632

THEORY AND PRACTICE OF ENGINEERING.

Hook II.

North ami South Wales, it is a calcareous mass, interposed between the old red sandstone,or where this is wanting, the more ancient slate rock below, and the sandstones and shalesof the coal above. The Derbyshire or Encrinital marble is formed of crinoidcal remains,composed of stems and detached ossicula of one species of Encrinite.

It is in the mountain limestone that the principal lead mines are situated ; in Derbyshirethe metal occurs in numerous veins which traverse the rock, and extend in some instancesinto the ancient volcanic bed. The perpendicular or rake veins are from 2 to 40 feet wide,and others are chasms or hollows in the rocks, several hundred feet wide, which also containmetallic ores and spar; manganese, iron, copper, zinc, &c., are also found, but that whichmost predominates is galena, or the sulphuret of lead; it is accompanied with fluor andcalcareous spar, sulphate and carbonate of barytes, iron pyrites, &c. &c.

Millstone Grit is a siliceous conglomerate, or quartzose sandstone, and is composed of thedetritus of the primary rocks. Fragments of granite, from the size of a pea to a largepebble, arc cemented together by a crystalline paste.

Old Red Sandstone: this is of enormous thickness, and has been stated at 10,000 feet; itconsists of many varieties and alternations of tile, stone, limestone, marl, conglomerates,shales, and sandstones ; the latter of various states of induration, which, when schistose, areemployed for roofing. The conglomerates contain abundance of quartz pebbles ; the redcolour predominates in the cementing material and the marls, and is derived from theperoxide of iron. The formation of the stratas has evidently resulted from the waste anddegradation of the ancient slate rocks, the detritus being cemented together by red sand ormarl into coarse conglomerates.

Among the organic remains are ancient forms of the Terebratul®, Spiriferaj, Product®,&c. ; Nautili, Ammonites, and Orthocoralite, &c ; Fishes, Fuci, &c. ; some of the fishesbelong to the genus Cephalaspis and Onchus.

Among the tile-stones the Ichthyodorulites of the genus Onchus has been found, and aspecies of Dipterus, with Mollusca of the genera Avicula, Area, Cucull®a, Terebratula;,Lingula, Turbo, Trochus, Turritella, Bellerophon , Orthoceras, and others.

Upper Silurian is chiefly found in Shropshire , Radnorshire, and Herefordshire , par-ticularly at Wooltiope.

Upper Ludlow Rocks arc formed of a grey thin bedded limestone, slightly micaceous.

The Aymestry Limestone is sub-crystalline, and either a grey or blue argillaceous limestone,highly fossiliferous; among the most numerous of the fossils is a species of Brachopoda,viz. the Terebratula navicula.

Lower Ludlow Rocks are composed of sandy shales and flags, with concretions of*,earthylimestone. The organic remains found in them are Corals , Terebratula leptena or pro-ducta, Ortliis, Pentamerius Knightii, Lingula, Orbicula, Bellerophon , &c. &c.

JVtnlock Formation. — The limestone of this formation is of a grey and blue colour, highlyconcretionary and subcrystalline. The shale is argillaceous, of a dark liver colour, withnodules of earthy limestone, and sometimes micaceous. The organic remains are chiefly ofthe lower order of marine animals, as Corals and Crinoidea, among which are Productsdepressa, Spirifera lineata, Euomphilus rugosus, Orthoceras annulatum, Consularia qua-drisulcata, Calymene Blumenbachii, (variolata, Asaphus caudatus).

Lower Silurian . — Caradoc Flags are thin-bedded impure shelly limestones, finelylaminated, slightly micaceous greenish sandstone.

The Sandstone is thick-bedded, and a freestone of various colours, as white, red, green,and purple. Tire grits are quartzose and conglomerate, the limestones sandy and gritty.Among the fossils are a few Crinoidea, Pentameris levis, Orthis, Terebratula, Leptama,Nucula, &c.

LumdiUo Flags are dark-coloured, ami chiefly calcareous, with some sandstone and schist;the fossils are several varieties of Trilobites, Asaphus Buchii, &c.

The Silurian system is composed of sedimentary deposits, which are either calcareous,arenaceous, or argillaceous ; the latter rocks are less indurated and less complicated intheir joints of cleavage, retaining in many places their original lamination. The arenaceousrocks take the character of ordinary sandstone and conglomerate, and the calcareous areonly partly crystalline. The whole of these rocks seem to have accumulated in a regularand tranquil manner, as they all show the lamina; of their deposition: in the sandstone thebeds are distinctly marked; in the limestones are evidences of regular stratification, thoughnodular and concave on their surfaces, and sometimes partially lenticular, indicating thattheir origin may have been similar to that of the coral reefs.

The joints and fissures which occur in the Silurian system are usually at right angleswith the planes of stratification ; the organic remains are only those of Invertebrata , 500 or600 species of shells, and 14 or 15 species of plants ; each of the four formations of this systemcontains distinct and characteristic species of fossils.

Cambrian System . — Upper Cambrian , or Plynlymmon Rocks. — In Cumberland theyarc formed of a dark limestone, containing corals and shells; in Wales of beds of conglo-merate, greywacke and greywacke slate, &c. The green slates and porphyries rest upon