IS (3.] 



Transition Rocks of Jrerner, 



417 



stone may be considered as rather of rare occurrence. Gray- 

 wacke and graywacke-slate are with us the priricipal members. 

 The first of these is a otone usually of a bluish colour, passing 

 into grey, and sometimes into greyish red; it is composed of 

 frae^ments, often of considerable size, but sometimes so minute 

 as .o be scarcely distinguishable; these fragments are quartz, 

 clay-slate, flinty-slate, and occasionally jasper, which are agglu- 

 tinated by a basis of clay-siate, through which minute particles 

 of mica are also sometimes dispersed. 



Graywacke-slate differs from the fine grained graywacke only 

 in its minute stratification and fissile character ; it bears so strong 

 a resemblance to clay-slate in hand specimens that even an ex- 

 perienced eye cannot distinguish it : in the rock it is not so easily 

 mistaken; it usually alternates with graywacke, and is often 

 remarkably contorted. Both substances are traversed by quartz 

 veins, which are sometimes of enormous dimensions, but gene- 

 rally very minute and abundant. 



The only limestones of this class that I know of are three: 

 first, that of Rae Quarry, near Crook, in Peeblesshire, where 

 it is interstratified with graywacke, and contains abundance of 

 shells. The second is that of Cumberland, on the lakes of 

 Windermere and Coniston, which also contains organised bo- 

 dies. The third is the Plymouth limestone, which, according 

 to the account of Professor Playfair, corroborated by Dr. Berger, 

 is also transition limestone; and in it Mx. Playfair states that he 

 found a petrified shell.* I have not myself visited the spot, but 

 it is of consequence to observe that the limestones of all these 

 different districts exhibit traces of organic remains. The other 

 transition rocks are trap and flinty-slate ;t but I have had no 

 opportunity of observing either of them in their natural position. 

 Such, according to Werner, is the extent of the transition 

 series; but it does not comprehend all the rocks which occur in 

 some of the transition districts, particularly that of Cumberland, 

 although, with little exception, it is adapted to the south of 

 Scotland in a very remarkable manner. 



I may now notice the extent of country occupied by rocks of 

 this description ; but such is our limited acquaintance even with 

 our own island that it can be done only in an imperfect manner. 

 We know too little of the north of Scotland to be able to say 

 what rocks occur beyond the Moray Frith ; but it is by no means 

 improbable, that when these regions have been more fully exa- 

 mined the transition series will be found among them. Indeed, 



* Illustrations of the Huttonian Theory, p. 165. 



f I suspect both these abound in the mountains of Cumberland, from speci* 

 aoens I have picked up among the loose fragments. 



Vol, I. VL 2D 



