By a rock formation, Mr. Jameson un derstands " a determinate assemblage of similar or dissimilar rock masses, which are characterised by external and internal relations, as an independent whole, that is, as an unity in the series of rock for mations." When the mass is uniform throughout, it constitutes a simple for mation ; but when dissimilar, a compound formation ; and when these formations are repeated, the whole is denominated a se ries or suite of formations. When indi vidual beds occur in different principal formations, as primitive trap in gneiss, mica-slate, &c. forming single indepen dent wholes, which always continue the same, notwithstanding the difference of rocks in which they are imbedded, and still form members of a series of forma tions, they are considered as independent formations. The inclination of a stratum. is the angle which the stratum forms with the horizon, and is determined by the quadrant. The dip is the point of the compass towards which the stratum in clines. The direction is the angle which the stratum makes with the ictian, and is determined by the compass. It is al ways at right angles to the dip.
The primitive rocks are chiefly com posed of substances, which chiefly sist of the siliceous and argillaceous earths. 1. Granite, the moor-stone of Cornwall, is a granular rock composed of felspar, mica, and quartz, united in vari ous proportions.—Schorl, garnet, tin stone, adularia, chlorite, and rock crystal, are among the accidental minerals which occur in this rock : it sometimes exists in large distinct globular, and sometimes in columnar concretions : it is sometimes stratified, but seldom contains any foreign beds. 2. Gneiss, is a stratified rock, formed of the same component parts as granite, but the mica exists in larger pro portion than in granite : it sometimes contains schorl, and, but more rarely, garnet and hornblende : its structure passes from that which approaches to the granular structure of granite, to the un dulated, and even the slaty structure. It is very frequently metalliferous, there be ing few metals which are not found in it. 3. Mica slate is likewise a distinctly stra tified rock, which rests on gneiss : it is composed of mica and quartz, disposed in a slaty structure : it frequently contains garnets, and sometimes hornblende, scion!, and tourmaline, kyanite, rutile, and felspar. Like gneiss it is frequently metalliferous, the ores generally occur ring in beds ; whereas in gneiss the ore is most frequently found in veins. 4. Clay slate is a simple rock, and follows the foregoing in the series of primitive rocks: it sometimes contains schorl, tourmaline, garnet, horneblende, chiastolite, and ac tynolite. There appear to be four differ ent kinds of clay-slate, chiefly distin guishable by their colours : yellowish grey, which connects clay-slate with mica slate ; dark and bluish grey, used as roof slate ; greenish grey, and lastly bluish and reddish grey, containing a few scales of mica. The rocks peculiar to this forma tion are, whet-slate, roof-slate, chlorite slate, talc-slate, alum-slate, drawing-slate, pot-stone, and flinty-slate. This, like those already mentioned, is a widely ex tended rock, and is also one of the most metalliferous. 5. Primitive lime-stone, is a simple mountain rock, which is more or less distinctly stratified, and is frequently metalliferous ; its colours are various, and its structure is always granular. Quartz and mica frequently occur in it accident ally : it also sometimes contains horn blende, actynblite, asbestos, serpentine, talc, steatite, tremolite, garnet, calcare ous spar, and slate-spar. 6. Primitive
trap, is a mountain which seems intimate ly connected with clay-slate. The term trap had been long used without a defi nite signification ; but \Verner has re sricted its application to rocks, principal ly containing hornblende, and black iron clay ; the iron-clay first appearing in the transition, and increasing in rocks of the newer periods. There are three distinct species of primitive trap. Common horn blende rock, under which are comprised, hornblende rock, and horneblende slate. Hornblende with felspar, a subordinate kind of which is greenstone, which has the following varieties : common green stone, a granular aggregate of hornblende and felspar. Porphyritic greenstone is the former, containing crystals of felspar. Greenstone porphyry, is the black por phyry of the ancients ; crystals of felspar are here also included, but the granular structure of the basis is hardly discover. able. Green porphyry, in which the gra nular structure is no longer visible, and crystals of compact felspar are included. The second species of primitive trap is greenstone slate, composed of horn blende and compact felspar, arranged in a slaty structure ; and the third is an inti mate mixture of hornblende with felspar, including mica in scales. It is found in beds in gneiss and mica-slate. 7. Ser pentine is a simple mountain rock, indis tinctly stratified. A great variety of other mineral bodies are found in it, and it is sometimes indeterminately mixed with limestone, forming what is termed verde antico. 8. Porphyry is a compound rock, formed of one substance in the form of grains or crystals, imbedded in another as its basis. The base is clay-stone, horn stone, compact felspar, pitch-stone, pearl stone, or obsidian : the imbedded crystals are of quartz or felspar. There appears to be two formations of porphyry ; the oldest consists principally of hornstone and felspar porphyry ; and the newer of clay, pitchstone, pearl-stone, and obsidian porphyry. 9. Sienite is a compound, granular, aggregated rock, formed of fel spar and hornblende, and sometimes con taining quartz and black mica. The horn blende distinguishes this rock from gra nite ; but the felspar, which is almost al. ways red, and seldom inclining to green, is the most abundant and essential por tion of the rock ; a circumstance which distinguishes it from greenstone, in which the felspar predominates, and is of a greenish colour ; whilst in sienite it is red or reddish. 10. Topaz rock is composed of quartz, topaz schorl, and a small por tion of lithomarge : the stratification of this rock is uncommonly distinct. 11. Quartz rock is a simple mountain rock, composed of small and flattish granular distinct concretions. This, as well as the former rock, is not very frequently met with, nor is of considerable extent. 12. Primitive flinty-slate is a simple rock, of which there exist two subspecies ; corn mon flinty slate, and Lydian stone. It is met with in considerable beds in clay slate. 13. Primitive gypsum is a simple rock, which is distinguishable from the newer gypsum, by its being mixed with mica and clay-slate. 14. White stone is a rock, which is sometimes of a slaty, and sometimes of a granular structure, and is chiefly composed of compact felspar, with a small proportion of mica.