ROCK. 



a. cement, as in sand-stones, pudding- 

 stones, and brecciee ; or are aggregated, 

 being composed of parts existing original. 

 ly on the spot where the masses are now 

 found, and connected together by their 

 original structure. They are also consi- 

 dered as simple aggregated, when one, as 

 a base, includes the other ; when the con- 

 tained portion is in the form of grains or 

 crystals, the structure is termed porpby- 

 ritic ; and when of a vesicular form, amyg- 

 daloidal : the double aggregated is when 

 a smaller structure is contained in a larg- 

 er, as granular slaty ; or when the two 

 structures exist near or beside each other; 

 when the one not including the other is 

 pointed out by a conjunction, as porphy- 

 ritic and amygdaloidal. 



The mountain masses themselves are 

 either of a stratified or of a seamed struc- 

 ture. When the masses composing the 

 rock are of one species, and disposed pa- 

 rallel to each other, those masses are 

 termed strata ; but when the mountain 

 mass is composed of different species of 

 rocks, thus disposed, it is said to be 

 formed of beds. In the seamed struc- 

 ture, the seams of stratification, though 

 parallel in one direction, intersect each 

 other in another. The masses thus divid- 

 ed by these intersecting seams, may be 

 considered as distinct concretions, and 

 may be instanced in the columnar formed. 

 Straight and much thicker masses are 

 also thus formed, by what is termed the 

 tabular seamed structure, and large 

 globular masses result trom the large 

 globular or massive structure. 



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 \vhichthestratumformswith 

 the horizon, and is determined by the 

 quadrant. The dip is the point of the 



compass towards which the stratum in- 

 clines. The direction 'M, the angle which 

 the stratum makes with the meiidian, 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 con- 

 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 verv 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, 

 schorl, 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- 



