Chap. t.

GEOLOGY.

633

the Skiddaw rock, and for the most part contain crystals of ehiastolite and hornblende,without any fossils.

Lower Cambrian consists of slates of various colours, with their cleavage at right anglesto their stratification ; they are found with conglomerates, porphyry, and greenstone, amongwhich are a few organic remains. In the rocks of this system are the first traces of fossilremains, and among them the first evidences of organic life ; those found belong to thegenera Cyathophyllum, Terebratula, Spirifera, Leptama, or Producta. Some of theCambrian and Welsh rocks have a mechanical origin : they contain marine organic remains,and have evidently been deposited by water ; but it is extraordinary that they are not onlystratified, but have cleavage planes inclined at a very considerable angle to the planes of thestrata, and are never coincident, although in some cases they seem almost parallel with them.In Wales the cleavage planes occasionally dip towards the same point of the compass asthose of stratification, but more generally in an opposite direction. The joints are un-doubtedly natural fissures, which traverse these rocks in straight and well-defined lines,the whole mass being frequently split by them into regular and symmetrical shaped blocks,which affords great facility in removing them from the quarry. These natural fissures havebeen produced since the deposition of the strata.

Slate is an argillaceous deposit, and the clay slate resembles in a great degree decomposedfelspar, which has been deprived of its potash by the action of water, and under particularcircumstances powdered blue slate, acted upon by great heat and an alkali, has been trans-formed into white and glassy crystalline grains of felspar.

In greywacke slate the lamina? of deposition show, on all the vertical planes, being parallel,or nearly so, to the plane of stratification, which differs from the clay slate, for their lamina,as has been stated, cross the planes of stratification, so that it may be split almost indefinitelyinto thin plates, in a nearly vertical direction; there are instances where the lamina? ofdeposition remain in clay slate; across the cleavage plane arc often seen stripes of colour,different from the mass, which arc evidently the marks of deposition interrupted by water.The most cleavable slate rock in the quarry shows a stratified deposition, although it isperfectly crystalline in its regular structure.

Cleavage has been thought, and probably is, the result of some agency after the sedimentwas deposited; it is most perfect in the ancient argillaceous strata, where the rocks arc of thefinest grain and uniform in their character. Heat operating upon argillaceous sediment, soas to overcome the natural horizontal lamination, is supposed to have induced a new, almostcrystalline fissility in vertical or highly inclined planes, having one general direction. Thelower slates are universally cleavable, whilst the upper are only partially so, and the polarityof the cleavage is the result of some general agency, which no doubt directed the molecularattraction.

Gneiss System. — The materials which compose these rocks are siliceous, argillaceous, andcalcareous, in a different state of aggregation to those before described. The siliceousstrata is composed of the same materials as those found in the secondary sandstones, quartz,felspar, and mica, but are not so worn by attrition, and bear some resemblance to granite.

The Argillaceous Strata do not much differ from the common clays, but from theirindurated character must have undergone different changes ; the prevalent colours are blue,red, purple, grey, and yellow; the green varieties contain chlorite.

Primary Limestone is met with in irregular beds, alternating with all the members of theprimary series; it has a highly crystalline texture, is compact, and both large and finegrained ; the purest and whitest are sometimes called saccharine limestone. In themountains at Carrara it is abundantly found, and contains no fossils ; it was once supposedthat this marble was formed before the existence of organic beings, but it has been provedto be a mere change of the limestone of the oolitic period. The calcareous rocks aroundthe bay of Spezia contain abundant fossils of the oolitic system, and exhibit a difference ofcharacter in proportion as they have been acted upon more or less by the Trappean andPlutonic rocks.

Quartz Rock , being divided by natural joints, breaks into rectangular or rhomboidal forms;its structure is rarely compact and crystalline throughout; it is sometimes mixed withfelspar, and sometimes with mica; it is of a white colour, but when impure it is eitherred or yellow.

Talcose Schist consists of talc alone, or quartz and talc ; it is found in thin beds, and oftenpasses into argillaceous schist.

Chlorite Schist is distinguishable by its green colour, and is saponaceous to the touch; itschief ingredients are chlorite and quartz, sometimes mixed with felspar and hornblende. Itis most frequently found with mica schist, into which it passes.

Hornblende Schist is chiefly composed of felspar and hornblende, containing occasionallygrains of quartz ; it is rarely met with in large masses, but is usually associated withgneiss.

Mica Schist is a crystalline compound of quartz and mica in different proportions ; itstexture is foliated or laminar, and it may sometimes be split into coarse plates. When it