ROCK. 



from the form of the bricks or ftones of which it is 

 condrufted. The internal drufture of fimple minerals 

 belongs properly to the department of the mineralogift, 

 and that of compound rocks, with their external ftruc- 

 ture, to the geologift. Compound rocks are either 



Granitic, compofed of grains or cryllals, united without 

 a cement, as in granite. 



Po.phyritic, confiding of a compaft ground, in which 

 diftinft cr^ftds are imbedded ; or of a granitic ground, in 

 which fome of the cryflals are much larger than the reft. 



AmygdMoidal, from the Latin amygdala, an almond, con- 

 taining kernel-fhaped cavities, filled with other mineral fub- 

 ftances. 



Conglomerated, compofed of fragments, or rounded ftones, 

 ^mented together, as in breccias and pudding-ftone. 



Granular, compofed of fmall grain?, either cemented, or 

 adhering, as in fand-ftones. 



The external ftrufture of rocks is either 



Stratified, compofed of layers or llrata. 



Tabular, in large plates : this includes the flaty ftrufture in 

 the mafs. 

 ■.•j Columnar, in regular columns or prifms. 



Globular, in fpherical mafies. 



Indeterminate, which includes all unllratified rocks that 

 have no determinate (hape. 



Stratified mountain?, or rocks, are compofed of layers or 

 llrata lying over each other, and divided by parallel leams, 

 like the leaves of a clofed book. In the feams or partings 

 which divide the ftrata, there are frequently thin laminx of 

 foft earthy matter, but fometimes the Surfaces of the upper 

 and lower ilratum are clofely joined, and require consi- 

 derable force to cleave them afunder. The layers are denomi- 

 nated ftrata. See Strata. 



It has been admitted, without fufficient evidence, that all 

 Uratified rocks were formed of the fediment of turbid water, 

 which arranged them in fuccefiion over each other, as the 

 muddy waves of the ocean depofit their contents in regular 

 layers upon the fhore. This mode of formation is called 

 mechanical depofition. It has alfo been generally believed, 

 that all rocks divided by parallel feams were formed mecha- 

 nically by the aftion of water, and as fuch layers occur oc- 

 cafionally in cryftalline rocks, it was inferred by fome 

 geologifts that this was a proof that fnch rocks had 

 been formed by water. Thus the tabular ftrufture, which 

 confifts of tables or plates of rock that have generally 

 a vertical direftion, and frequently a flaty cleavage, has 

 been confounded with Gratification : this has given rile to 

 much confufion ami contradiction in the defcription of rocks ; 

 fome geologifts denying, and olhers aliening the Gratification 

 of the fame rock. The tabular and flaty ftrufture of many 

 rocks may, with as great probability, be attributed to cryftal- 

 line arrangement, as the lamellar ftrufture of a cryftal of fel- 

 fpar, or a plate of mica. The laws of cryftallization have but 

 recently arreftedthe attention of philolophers, and their re- 

 fearches have been principally confined to their effefts in a 

 fmall fcale. The cryftallization of mountain mafles is 

 equally deferving of notice, as to this caufe muft we refer 

 both their tabular, columnar, and globular ftrufture. 



The columnar ftrufture confifts of regular columns or 

 polyhedral prifms, and is almoft peculiar to trap or bafaltic 

 rock?, and to volcanic rocks : thefe fometimes form vaft 

 ranges of natural columns, as in Iceland, the Liipari iflands, 

 the Motta in Sicily, the Giants' Caufeway in Ireland, and 

 at Staffa. It was long contended againft all probability, 

 that thefe columns were the effects of accidental rents occa- 

 sioned by the drying or fhrinking in of the mafs ; but their 

 Vol. XXX. 



regularity, and (in many inftances) the convex and concave 

 articulations of the joints, prove that they are effefts of cryf- 

 tallization. Thefe columns have been obferved by Col. Imrie 

 and others formed in currents of lava, that could be traced 

 to the craters of volcanoes. Tranfaftions of the Wernerian 

 Society, vol. ii. pt. I. 



The globular ftrufture confifts of balls, fometimes de- 

 tached, at other times imbedded in rocks of the fame kind. 

 Thefe balls are frequently compofed of concentric fpherical 

 layers. This ftrufture is not unfrequent in bafaltic and 

 granitic rocks. The balls are generally harder than the rocks 

 in which they arc imbedded, and frequently retain their fhape 

 after the outer rock is decompofed. Inftances of this ftrufture 

 occur in the bafaltic rock of Staflord(hire,'callcd Rowley Rag. 

 (See Rowley Rag.) Globular diftinft concretions of granite 

 are from one to two or more fathoms in diameter. Thefe con- 

 cretions are again compofed of curved lamellar concretions, and 

 always include a harder central mafs. The fpaces between the 

 globes are filled with granite pod'efling lefs folidity, which 

 decays more readily, and thus leaves the harder central mafles 

 heaped on each other, or drewed about. Thefe diftinft concre- 

 tions muft not be confounded with rolled mades. Beautiful 

 examples of thefe concretions occur in the ifiand of Arran, 

 Bohemia, the Hartz, the Fichtelgebirge, and other places. 



The indeterminate or amorphous ftrufture appears to be 

 the mod common in unllratified rocks ; but our confined 

 and partial obfervations may frequently lead us to conclude, 

 that rocks have no determinate ftrufture, whereas, could we 

 afcertam their arrangement throughout their whole extent, 

 we might perceive that their drufture, viewed on a great fcale, 

 was as regular as that of many Uratified rocks. 



The pofition of rocks, with refpeft to each other, is an in- 

 terefling fubjeft of geological inquiry, as it is fuppofed to de- 

 termine their relative ages, and to elucidate the mode of their 

 formation. When one rock covers another in fuch a manner 

 as to appear moulded upon it, having the fame elevations and 

 depreffions, it is faid by the German geologifls to lie in a con- 

 formable pofition. In this manner the fchidofe or flaty 

 rocks frequently cover rocks and mountains of granite. 

 Stratified rocks are alfo generally conformable to the fhape 

 of the lower rocks, except in fituations where the ftrata 

 appear to have been broken by fome fubfequent convulfion 

 ot nature, which has deranged their original pofition, and 

 occafioned them to abut againfl each other, or has given the 

 upper ltrata a contrary inclination to the lower. In this 

 cafe their pofition is faid to be unconformable. Where 

 mafles of unflratified rocks cover other rocks, filling up the 

 cavities, and lie without any conformity to the fhape of the 

 low;er rocks, they are called unconformable and overlying. 

 Tins pofition is common to moll rocks of volcanic origin, and 

 to bafaltic rocks ; in the former there can be no doubt re- 

 fpefting the mode of their formation. Streams of liquid 

 lava pouring through vail tiftures and openings have covered 

 the inequalities of the lower grounds, filling up vallies, and 

 accumulating as the lava cooled and conlohdated, thus 

 forming jmmenfe mounds and abrupt precipices : fome geo- 

 logifts afcribe a fimilar formation to bafaltic and porphyritic 

 overlying rocks. 



Thofe rocks which are commonly imbedded in other rocks, 

 arefaidtobchibordinate formations ; thus granular lime- done, 

 ferpi ntine, and hornblende, frequently form beds in fchillofe 

 rocks, fuch asgneifs, mica-flatc, and clay. (late, and arc then 

 defcribed by geologifts as Subordinate to the latter. 



Bcfidc the above politions, the continuity of rocks is fre- 

 quently broken by vertical feams or Allures filled with mineral 

 matter different from that of the rock which they interfeft. 

 3 D 



