8
fine dust and skeleton crystals, as well as slendermicrolites of feldspar and augite. Slender needlesof apatite occur included in the feldspar. Theanalysis which follows shows the Yakima basalt tobe closely related chemically to the Teanawaybasalt. It is much less basic than typical basalt,and would be termed a vaalose in the more exactquantitative classification.
Per cent.
Si0 2 . 54.50
A1 8 0 3 . 14.43
Fe 2 O s . 2.17
FeO. 8.80
CaO. 8.01
Na 2 0. 3-05
K 2 0. 1-29
H 2 0 at 105°.29
H s O above 105°. 1.09
TiO s . 1-69
Zr0 2 . none
C0 2 . none
P 2 0 5 .21
S0 3 .11
NiO. none
MnO..,.10
BaO. 06
SrO.09
Occurrence. —In the southwest corner of theMount Stuart quadrangle are two small areasof diabase. The larger of these is on the dividebetween Manastash and Taneum creeks andforms an irregular mass intrusive in both theTaneum andesite and the Manastash sandstone.The other occurrence is on the western edge of thequadrangle, being part of a large mass in the adja-cent area.
In this vicinity there are several large dikes ofdiabase which cut the same formations as theintrusive masses just described. The connectionof these dikes with the other diabase is very prob-able, since one can be traced to its junction with thelarger mass. The largest of these dikes occurs onthe west side of North Fork of Manastash Creekand is unique in that it cuts the lower sheets ofYakima basalt. This occurrence, together with thegeneral distribution of the diabase, justifies the con-clusion that the diabase originated from the samemagma as the Yakima basalt, the larger masses ofdiabase representing the intrusive bodies of moltenrock which connected upward, through conduitsnow indicated by the dikes, with the lava flows atthe surface. As has been shown in the Ellensburgfolio (No. 86), Bald Mountain, which is immedi-ately south of the Mount Stuart quadrangle, wasan important center of volcanic activity during theeruption of the Miocene basalt flows.
Description. —The diabase is dark-brown rock,with medium grain, and the diabasic or ophitictexture is plainly exhibited, especially on theweathered surface. The rock is hard and with-stands erosion well, the outcrops being commonlyrounded but generally projecting above the rockswith which the diabase is in contact. The dikeswhich cut the Manastash sandstone are readily dis-tinguished and can be traced for short distanceseven where rock waste covers the surface generally.
The constituents which can be detected mega-scopically are pyroxene and feldspar. Under themicroscope the rock is seen to be composed ofplagioclase, augite, hypersthene, olivine, apatite,and magnetite. The plagioclase, chiefly labra-dorite, is the most abundant constituent, and thecrystals are often zonal. The augite is green orbrownish, with a faint violet tinge. The hyper-sthene occurs in phases of the diabase in whicholivine is wanting, and, when unaltered, forms stoutprisms or anhedral grains. The olivine is lessimportant than the augite and is best developed inthe diabase of the dike in the Yakima basalt.This rock shows the order of crystallization to havebeen apatite, plagioclase, olivine, magnetite, and,last of all, the augite, which forms large individ-uals, often a centimeter in diameter. In some thinsections the olivine is found altered to typicalbrown iddingsite with lamellar structure. It isprobable that some hypersthene has been replacedalso by iddingsite. The apatite occurs in longneedles, often grouped in bundles.
ELLENSBURG FORMATION.
Occurrence. —Although the Ellensburg formationhas an areal extent of nearly 100 square miles
within this quadrangle, it is not at all conspicuous.Over the greater part of Kittitas Valley a thickmantle of alluvium conceals the sandstone and con-glomerate of this formation. The best exposuresare along the bluffs overlooking the river betweenDudley and Thorp. Another locality where atypical section of the Ellensburg formation can beseen is immediately east of the Normal School atEllensburg, where this formation stands above thegeneral valley level. Elsewhere the soft characterof the formation renders it easily eroded, so thatsurface wash usually conceals the undisturbed rockbelow.
Two smaller areas, separated from the KittitasValley areas, occur on the southern slope of Look-out Mountain and northwest of Horse Canyon.The latter exposure measures only 30 feet in thick-ness, representing the basal beds of conglomerateand tuffaceous sandstone resting on the basalt. OnLookout Mountain a square mile or more of thisformation is preserved, but even here only a slightthickness remains.
Description, —The Ellensburg formation com-prises light-colored sandstones and conglomerates,which are so friable and loose textured as to deserveoften to be termed simply sands and gravels. Thedistinctive characters of the formation are itsmarked variations in grain, the common occurrenceof pumice fragments, and the prevailing crossstratification or stream bedding. These make itreadily distinguishable from the older sedimentaryformations of the region.
The Ellensburg formation is composed largely ofvolcanic sediments, which are of foreign origin.Pebbles or bowlders derived from the underlyingbasalt are only rarely seen, the conglomerate bedsbeing composed of pebbles of light-gray and purplehornblende-andesite and of white pumice of thesame composition, while the sandstones and shalesof the Ellensburg formation consist of finely com-minuted andesitic material, which represents inpart the volcanic dust from explosive eruptions.The lava from which these pebbles and bowlderswere derived is not exposed within the MountStuart quadrangle, but undoubtedly occurs in themountains to the southwest.
The number and thickness of the conglomeratebeds and the prevalence of stream bedding indicatethat the formation is largely the result of fiuviatilerather than lacustrine conditions of sedimentation.South of Horse Canyon are angular bowlders ofandesite measuring several feet in diameter whichcome from a conglomerate in the Ellensburgformation. The transportation of such materialcould have been effected only by powerful streams.The original thickness of this formation can not bestated. Along Yakima River several hundred feetof Ellensburg beds are exposed, while a well sunkin Kittitas Valley penetrated about 700 feet withoutreaching the base of the formation. Farther south,in the vicinity of North Yakima , the Ellensburgformation is known to be 1600 feet thick, so it isprobable that its original thickness in KittitasValley was at least 1000 feet.
The deformation to which these beds have beensubjected has been slight. The elevations at whichthe basal bed is found on Lookout Mountain andon Dry Creek indicate a low dip to the south,toward the center of the valley. Beyond the limitsof this quadrangle the Ellensburg sandstone isknown to dip toward the middle of the valley,so that Kittitas Valley is coincident in position witha gentle flexure, forming a basin whose longestdiameter measures over 30 miles, from northwest tosoutheast. The occurrence of the Ellensburg for-mation northwest of Horse Canyon may be due toa slight fault which has thrown this bed downsufficiently to protect it from erosion.
Flora. —Fossil plants have been found in theEllensburg sandstone at a quarry just beyond thesoutheast corner of this quadrangle. This localityalso yielded a few teeth of Hipparion, a Miocene representative of the horse family.
The following report on the fossil plants fromthis locality has been made by Dr. F. H. Knowlton:
So far as known, the first collection of fossil plants made inthe vicinity of Ellensburg, Wash., was obtained by Mr. J. S.Diller in the spring of 1892. This is a small collection,embracing only half a dozen pieces of matrix, and was madeat a point about 6 miles southeast of Ellensburg. It containsseveral species, the most abundant and characteristic beingPlatanus dissecta Lesq.
In 1893 Prof. I. C. Russell obtained from the same localitya considerable collection, in which I was able to recognize ten
species. I have recently studied this collection again, andpresent the following list of species:
Salix pseudo argentea Knowlton.
Populus glandulifera Heer.
Populus russelli Knowlton.
Alnus sp.
Ulmus californiea Lesquereux.
Ulmus pseudo-fulva Lesquereux.
Platanus dissecta Lesquereux .
Platanus aceroides? (Goppert.) Heer.
Diospyros elliptica Knowlton.
Maguolia lanceolata Lesquereux.
The matrix of the specimens is a white, genet ally finegrained volcanic ash, identical in appearance with that fiomVan Horn’s ranch (Mascall beds) in the John Day basin,Oregon .
Of the 10 species above enumerated 6 are found in greateror less abundance in the Mascall beds, and I do not hesitateto refer the Ellensburg material to this horizon. The Mascallbeds are regarded as being upper Miocene in age.
It may be noted that no formation lias beenfound in this region equivalent to the John Dayformation (Oligocene ) of the Eastern Oregonsection.
RHYOLITE.
Occurrence. —East and west of Rvepatch thereare several areas of rhyolitic lava. This rockweathers white or a rusty yellow and only rarelyshows its true character when examined in the out-crop. In many places the rock resembles a fineshale that has suffered alteration from mineral-spring action. Microscopic examination of thisrock shows its rhyolitic character, both compactlava and tuff being present. The rhyolite containsscattered phenocrysts of bipyramidal quartz andangular fragments of the same mineral. Theground mass is composed almost wholly of crypto-crystalline aggregates of quartz and feldspar withwell-defined spherulitic intergrowths.
The relations of the westernmost and largestoccurrence of rhyolite appear to indicate that therhyolitic flow occurred at the close of the eruptionof Teanaway basalt. Elsewhere, however, the dis-tribution of the rhyolite, which directly overliesboth Roslyn sandstone and Teanaway basalt, affordsconclusive evidence that the rhyolitic lava flowedout over the eroded surface of these Eocene forma-tions, probably in Pliocene time, and in the west-ernmost locality simply conceals the Roslyn-Teanaway contact.
Another occurrence of volcanic rock may bementioned in this connection. In the extremenorthwest corner of the quadrangle the peridotiteis capped for an area a few yards in diameter witha breccia having all the characters of a volcanicrock. This rock is made up in its finer portions ofangular fragments of crystals of quartz, feldspar,and some ferromagnesian minerals. No similaroccurrence was observed elsewhere within thequadrangle, but it is very probable that this brecciais an outlier of the late Tertiary lavas that occur onthe western side of Clealum River in the adjoiningquadrangle.
Quaternary Deposits.
GL4CIAL DEPOSITS.
The glacial deposits of Yakima Valley aredirectly the result of the overloading of the streamsby the glaciers in the headwater tributaries, butpurely glacial deposits are not important in thisarea. Along Ingalls Creek the floor of the valleyis in places covered with immense blocks of rockwhich the stream is powerless to move, and a smallmoraine has shifted the lower part of TurnpikeCreek somewhat to the east. Small moraines alsooccur on Peshastin Creek below Ingalls Creek.
ALLUVIUM.
The general distribution of valley alluvium isshown on the areal geology map of this folio.Several of the principal areas will be described,and of these Kittitas Valley is the largest. Thisstructural basin has had its floor largely modifiedby stream erosion, and a thick mantle of streamdeposits covers the greater part of the valley.Along the valley margins the coarse detritus hasbeen derived from the basalt-covered slopes aboveand is very angular in character. The “scab-land”characterized by this material differs little from thesurface of the basalt plateaus covered with large andsmall fragments of disintegrated basalt, so that'theline between areas of alluvium and those of rockwaste which has not suffered transportation is notalways very definite. In other places are fine-
grained deposits which seem in part to lie of wind-blown material. Lower in the valley fine-grainedalluvium becomes of general occurrence.
In the upper valley of Yakima River, north ofthe basalt escarpment, there are thick deposits ofalluvium. On the flood-plain along the river coarse,clean gravels predominate, and there are areas ofsimilar deposits on the upper benches, so that theamount of agricultural land can not be determinedfrom the distribution of alluvium as mapped.Along the Teanaway the areas of alluvium outlinedon the areal geology map are chiefly bottom landof fine quality. Swank Prairie includes severalsquare miles of very fine alluvium, comparativelyfree from bowlders, so that the area is one especiallyadapted for wheat raising. The character of thescattered bowlders and the sections of stratifiedgravels afforded by a few wells indicate the truealluvial nature of the surface deposits over SwankPrairie. Another exceptional area of alluvium isCamas Land. Here a level prairie of several hun-dred acres with a rich loam has been preserved bythe gabbro barrier on Camas Creek.
A marked feature in the more extensive valleydeposits of this quadrangle is the occurrence ofwell-developed terraces. Below Clealum threeplainlv defined levels can he traced for severalmiles, and similar terraces extend up both Teana-wav River and Swank Creek even beyond thelimits of the alluvium shown on the areal geologymap. The highest of such gravel terraces markthe extent to which the streams filled their oldvalleys at the close of the Glacial epoch. Theextent of this filling is not wholly evident, sinceonly in a portion of their courses have thestreams cut away the gravels from the rock.Indeed, the results of borings made in the vicinityof Clealum show the presence of several hundredfeet of gravels and indicate that the rock floor ofthe valley at this point is somewhat lower thanthe river bottom in the basalt south of LookoutMountain. This feature may be due to landslidesat Lookout Mountain or it may indicate change'sin elevation.
Stream gravels and large bowlders from theupper Yakima occur at three different levels eastof Clealum Point, the highest of which is 3300feet, and at 2680 feet the bowlders form a distinctterrace. Similar gravels at about 2600 feet wereobserved west of Bristol. These high-level depos-its are evidence of stream work at an earlier stage,when Yakima River occupied a wide valley south-west of Lookout Mountain where now it is in acanyon.
ECONOMIC GEOLOGY.
GOLD.
HISTORY OF THE DISTRICT.
The three principal gold-mining districts ofcentral Washington are included in the MountStuart quadrangle. The Peshastin placers werediscovered in 1860 and have been worked inter-mittently ever since. The Swank placers havebeen worked rather more steadily since their dis-covery in 1868. Gold-bearing veins were first-located in the Peshastin district in 1873, and inthe Swauk in 1881. The mineral veins of theNegro Creek district constitute a continuation ofthose in the Peshastin district.
Mining in these districts has been conductedby small owners, and it is impossible to secure anydefinite data regarding production. The output ofgold of Kittitas County for the years 1884 to 1896,as leported by the Director of the Mint, aggregates$/64,16o. About $5000 of silver was reportedfiom that county for the same period. ThePeshastin district is now included in ChelanCounty, but during this period it was a part ofKittitas County. The years 1892 and 1895 wereseasons of maximum production, and the area prob-ably ^ would have steadily increased its outputhad it not been for the exodus of miners to Alaska .
n view of the activity in these districts in the yearspieceding 1884, as well as the production of thelast seven years, it seems that $2,000,000 would bea conservative estimate of the total gold production.
n the last five years companies with larger capi-tals have purchased the claims of the small oper-ator, and mining operations will now be conductedmore economically and probably with an increasein the gold production.