134 Dr. Mac Culloch's observations 



bright, and the texture so compact, that they can scarcely be dis- 

 tinguished from jasper, a rock which, I may remark, although 

 hitherto but little examined in its geological relations, possesses a 

 very near affinity to the family of porphyry, as well in the extensive 

 independence of its position among the regular rocks, as in its ap- 

 pearance and composition. 



The occasional minerals which are found in these rocks are 

 hornblende, quartz, and epidote, all of them entering into the com- 

 position of some of the varieties, and the latter in particular form- 

 ing a very conspicuous feature among them, being disposed either 

 in the form of veins, or in amygdaloidal cavities, or else in occasional 

 grains. Hard breccias, of which the structure can scarcely be detected 

 except on the weathered surfaces, and exactly resembling those so 

 conspicuous on Ben Nevis, also occur dispersedly among the more 

 simple rocks, the fragments consisting only of different varieties of 

 the same substance. 



Before quitting these rocks it will not be uninteresting to mark 

 the principal circumstances in which they differ from the analagous 

 rocks which occur in Arran, in the Ochil hills, and in many other 

 parts of Scotland. They all consist alike of claystone and compact 

 felspar, simple or porphyritic. But they differ in situation, the hills of 

 Glenco reposing on granite and the older schists, while the former 

 lie above the red sandstone. They also differ in their general 

 features, since the former assume a spiry shape, while the others 

 present a succession of tame and rounded outlines. They appear 

 equally to differ in durability, since although the hills of Glenco are 

 destroyed by the effects of the mountain torrents, they are not like 

 those of Arran subject to decompose by the ordinary action of the 

 atmosphere. In the variety of composition there is also a con- 

 spicuous difference, the infinite number of hard, coloured, and com- 



