486 CUMBRAY (LITTLE). GEOLOGY. 



stone.* Fragments of white sandstone are also found 

 scattered on the eastern shore ; derived probably from 

 some masses entangled in the overlying rocks, which do 

 not appear, or at least escaped my observation. 



The external general aspect of the trap rocks is various. 

 They often present a rude prismatic fracture, and occa- 

 sionally approach to a more perfect columnar structure. 



The predominant varieties bear a strong resemblance 

 to those of Mull ; consisting of clinkstone passing to clay- 

 stone, and displaying various intermediate stages of trans- 

 ition. They differ materially in aspect from the rocks 

 of the Garroch head, but correspond with those of Pen- 

 cross, and with an extensive range that forms the high 

 land of the adjoining coast. 



The colours are pale grey, dark grey, purplish brown, 

 and reddish brown ; and the predominance of the two 

 latter gives a very peculiar appearance to the whole, as 

 they do wherever they occur. On the opposite mainland 

 they often resemble in colour the sandstone with which 

 they are associated, so strongly, that it is impossible to 



distinguish the two rocks without a close examination. 



o 



Occasionally, the harder varieties split in a schistose man- 

 ner, forming a schistose clinkstone. 



The structure is occasionally porphyritic, and sometimes 

 amygdaloidal. There is nothing unusual in the porphyries, 

 but the amygdaloids present some peculiarities worthy 

 of notice. They frequently contain brown spar, often 

 forming very large nodules ; and it may be remarked that 

 this substance is of rare occurrence in trap, when compared 

 with the other minerals that occupy similar cavities. The 

 form of the amygdaloidal cavity is more interesting than 

 the imbedded substance. It is frequently elongated, and 

 sometimes contorted ; the surrounding solid parts also, 

 giving indications of a corresponding texture. Where the 



* Plate XXXIII. %. 2. 



