400 ARRAN. GEOLOGY. OVERLYING ROCKS. 



sometimes joined in such a manner as to resemble a 

 conglomerate rock, while, in other cases, ramifying veins 

 of the white are found passing through the dark variety. 

 It is not easy to explain this appearance, nor to deter- 

 mine whether these are real fragments or only acci- 

 dental modifications in the distribution of the substances 

 which form the greenstone. Real trap conglomerates 

 are also found associated with the other greenstones, 

 in the usual irregular manner in which they always 

 occur, and often presenting a considerably indurated 

 texture. 



The last and most rare variety of greenstone to be 

 noticed, is of a dark colour and coarse texture, strongly 

 resembling some of the rocks formerly mentioned as 

 found at Harris in Rum. It contains augit, and, gene- 

 rally also, compact mesotype (possibly nadelstein); this 

 latter compound resembling precisely the rock found at 

 Eilan Wirrey in the Shiant isles, and in Sky. It must 

 be remarked, that the mesotype does not here occupy 

 an amygdaloidal cavity, but is confusedly united to the 

 other ingredients, forming a regular constituent part of 

 the rock. It seems occasionally to pass into chalcedony, 

 into quartz, and into prehnite ; but the minuteness and 

 imbedded position of the globules prevents these changes 

 from being accurately examined. 



It may be added to this account of the varieties of 

 greenstone, that it occasionally occurs of a porphyritic 

 aspect ; and it is unnecessary perhaps to say, that basalt 

 is also found among the other rocks of this division; 

 sometimes in the overlying masses, but more frequently 

 in the veins. 



Having thus considered, in as much detail as appears 

 necessary, the several substances which form the ovef- 

 lying rocks of Arran, it remains to describe their geological 

 connexions with each other and with the stratified rocks ; 

 together with such parts of their topography as are either 



