G72

THEORY AND PRACTICE OF ENGINEERING.

Book II.

which has occasioned it to be observed, that the granite peaks rise like islands in an oceanof slate. The range or bearing of the masses of granite is about north-east and south-west,a mV the mines occur on both sides of it. Both the granitic tracts and the slates in theirvicinity are intersected by veins or dykes of a porphyritic felspar rock (provineially calledElvan). These veins have been traced for miles, passing uninterruptedly through bothgranite and slate ; their usual direction is about 20° south of west, anti they are generallyseveral fathoms in width : where they fall in contact with the veins they appear as if theyhad been portions of the strata.

The schistose varieties of the slate formation, considerably above the granite, containbeds of limestone, which coincide in position with the slaty lamina?, but are more generallyirregular and unconformablc. The metalliferous veins or lodes have an average directionof 4 degrees south of true west, but the general bearings are not the same in other partsof Cornwall : those of St. Just, for example, run about 35° north of west; in the samedistrict, and even in the same mine, (as at Doleoath, East Wheel Crofty, &c.), there arcoften two series of lodes, one bearing nearly east and west, whilst the others, called counter*lodes, are nearly south-east and north-west.

The dip or inclination of the lodes is about 60° or 70° from the horizon, and four out ofsix may be said to incline towards the nearest mass of granite; the lodes near Dart-moor arc for the most part flatter than those in the west of Cornwall . Taken on thewhole they appear tolerably straight in direction and in inclination, but when examinedin detail, it will be found that they exhibit almost continual curvatures or irregularities ;still, however, these flexures seem projected on certain lines, which have considerable con-stancy.

The width of the lodes on the average is about 3| feet, but they vary from a mere line to40 or 50 feet; each lode seems to have a natural or casual breadth of its own. Thecomposition of the lodes is as variable as the nature of the rocks through winch they pass ;the greater number is composed of earthy matter, of the nature of the contiguous rock,mixed with large quantities of quartz. Those ingredients are sometimes in separateveins, but for the most part are mixed without regularity or order; through them themetallic ores are dispersed, sometimes thickly, or in irregular lumps connected with eachother by small veins of ore ; in other cases the ore is very sparingly sprinkled throughthe earthy matter of the vein, and in some rare instances forms the larger part of itscontents. The masses of ore in the lodes usually dip from the granite, and the deepestparts of the mines are consequently farthest from where that rock appears at the surface.

There is a second series of veins which run nearly at right angles to the lodes, calledcross-courses when they are composed of quartz, and Jiucans when of clay. The generaldirection of the former is somewhere about south-east and north-west: their dimensionsare variable, being perhaps on an average 2 feet; their dip fluctuates, but as a generalrule, it is greater from the horizon than that of the lodes. It has been already mentionedthat quartz and clay form -the larger part of their ingredients : this clay is invariably ofthe same character as the contiguous rock.

Tin and copper ores are occasionally found in small quantities in the cross veins, and intwo or three instances silver and its ores have occurred to some amount. The chief metallicproduce of this class of veins is lead ore, but they seldom yield it in the neighbourhood oflodes which are productive of other metals ; it being a general law in Cornwall , thattwo series of veins, at right angles to each other, are seldom found productive in the samedistrict.

Both the lodes and cross veins ramify and divide, and whilst the one which is rich willsometimes within a short distance dwindle away, that which is small will often enlargeami become productive.

As these two varieties of veins run at right angles to each other, they ot coursefrequently meet and intersect. There are a few cases of the lodes traversing the crossreins, but in the larger number of instances the cross veins cut through the lodes, andoccasionally simply intersect them, but generally a displacement attends their contact;the separated portions of the lodes not occurring exactly opposite to each other on bothsides of the cross vein. These displacements are called heaves, and although they are usuallyfor a few feet or fathoms only, yet some cases are on record where the discordances are asmuch as 20, 30, or 40 fathoms, and in one instance 72 fathoms. It is not easy to lay downa rule for finding again the second portion, but it is perhaps rather more frequent todiscover it on the side of the obtuse angle formed at the intersection than on the acute. Itis obvious that on whatever portion of the lode we approach the cross veins, the otherwill be found towards the same hand ; the separated portions are more commonly foundtowards the right hand than the left. These heaves are the most intricate and baffling pheno-mena with which the Cornish miners have to contend.

'lliere is a third series of veins bearing parallel to the lodes, which arc generally of smallsize, and consist of clay, called slides. These are confined to the slate districts, and seldommetalliferous : they intersect the lodes on the lines of their inclinations, and cut off the lower