240 Outlines of Geology. 



bent upon slate, and even alternating in strata with that rock, and 

 gradually passing into it. The red rock prevalent about Exeter, 

 and that which occurs in the vicinity of Edinburgh, and which is 

 seen in perfection at Hawthornden, have been referred by some 

 geologists to this formation — the former probably does belong to 

 it, but the latter strictly appertains to the red marl. 



There occur in various parts of Britain a variety of sandstones, 

 some approaching the nature of breccias and conglomerates ; 

 others slaty, and others almost exactly corresponding to the red 

 marl in texture aud appearance, which, when beneath the 

 mountain limestone, and upon grauwacke or slate, are usually, 

 and properly enough, called members of the old sandstone family, 

 and in this way we may perspicuously dispose of them without 

 entering into the pedigree of the old red and of the new red, 

 which some geologists have descanted upon with such pertina- 

 cious prolixity. I may, however, here state, that sandstone does 

 not always intervene between mountain limestone and slate ; 

 on the contrary, numerous instances might be adduced of lime- 

 stone at once recumbent upon slate, in which case it is generally 

 very scanty in fossil remains, and often passes, by a kind of in- 

 sensible gradation, into the slaty rock. 



We here take leave of that series of rocks which Messrs. Phillips 

 and Conybeare, in their excellent Outlines of Geology, have ap- 

 propriately designated the medial order. 



The next substance we have to examine has received the un- 

 couth and unmeaning name of grauwacke ; it is a rock which has 

 often been regarded as setting definition at defiance, and some- 

 times specimens not referable to any other classes, and substances 

 of doubtful origin, have been indiscriminately considered as of this 

 family. A rock with more or less of a slaty texture, but distin- 

 guished from slate by its less?perfect lamellar fracture, and above 

 all, by its embedded fragments, and being at the same time es- 

 sentially argillaceous, appears to me to constitute legitimate 

 grauwacke. If I found evident fragments of slate embedded in an 

 homogeneous slate — or angular pieces of quartz ; or of chlorite 

 slate, in black slate ; or here and there an imperfectly-rounded 



