394 ARRAX. GEOLOGV. OVEULYING ROCKS. 



parison of distant points, regard being had to the dispo- 

 sition of the strata. This can sometimes be done even 

 in granite, although not a stratified rock, since it is con- 

 sistent in maintaining its unstratified character and its 

 inferiority of position. But the rocks under review are 

 sometimes disposed in shapeless overlying masses ; at 

 others in beds parallel to the strata, and either superior or 

 inferior in position, while in still other cases they inter- 

 sect them,* forming large intruding masses or smaller veins. 

 Hence arises that perplexity which prevents any accurate 

 inferences from one portion to another ; so that the account 

 of any mass can scarcely be extended beyond the imme- 

 diate point of observation. They are well known by 

 the several names of basalt, greenstone, syenite, claystone, 

 clinkstone, compact felspar, and porphyry. But these 

 rocks will all, I believe, be found to pass into each other 

 by regular gradations ; while the connexions of all of them 

 with the stratified rocks are the same. 



It will on the present occasion be convenient to invert 

 the usual order of description, and to examine the several 

 rocks and their characters before describing their places. 

 Much repetition will thus be saved, and if, as will fre- 

 quently happen, a claystone be found porphyritic where 

 I have represented it as plain, the reader who may follow 

 this description through Arran, will feel no surprise ; know- 

 ing that such variations are of frequent occurrence. 



I shall begin this series from claystone, as being perhaps 

 the simplest. This rock is not uncommon in Scotland, 

 since well characterized examples of it occur in the Pent- 

 land and in the Ochil hills. It is to be seen in Arran, 

 passing by various stages into compact felspar or into 

 felspar porphyry, but is riot found in any considerable 

 quantity in its simplest state. In the first state of indu- 

 ration however, it occurs abundantly, and under such 

 an aspect that mineralogists will perhaps rarely agree 



Plate XXIV. fig. 4. 





