Mr. Brtce on the Geological Structure of Roseneath. 115 



the nature of the sediment, or in the amount of heat or pressure, would be 

 sufBcient to produce the want of uniformity, and the variations from a 

 definite type, which we now observe. The general direction of the beds 

 is about north-east and south-west, the dip being to the south-east, at 

 angles varying from 40* to 70° ; — but there are many local exceptions. 



b. Plutonic ami Metamorphic Rocks. 



5. The slates above described contain subordinate beds and veins, 

 which possess considerable interest. 



In passing over the summit level of the road between Gareloch-head 

 and Portincaple Ferry, the observer who is accustomed to distinguish 

 rocks from a distance, by the peculiar forms which the different species 

 impress upon the surface, cannot fail to notice a remarkable ridge on the 

 right, projecting from the smooth outline of the hill side, and trending in a 

 straight line towards the base of the mountains. Its form, as seen from 

 the road, is represented in the annexed sketch, No. 1, which gives the 



a Ridge of porphyry, rising through the mica slate, 



b h The hill sides formed of mica slate. 



c Summit level of the road between Gareloch and Portincaple. 



d Mountain stream running along the junction. 



outline of the surface. It is obviously composed of matter erupted through 

 the slate, and on examination is found to consist of a highly felspathic 

 rock. The base of this rock is a vei7 compact mixture of quartz and 

 felspar, in which crystals of felspar and mica are imbedded, the latter 

 ingredient being constantly present, the former often wanting. The pre- 

 vailing colour is yellowish red, given by the felspar in the base. From 

 the prevalence of felspar, and tho mode in which the crystals arc dis- 

 seminated, the rock must bo called, according to the present views, a 

 felspar porphyry ; though the constant presence of mica as a constituent, 

 and the compound character of the base, seem rather to require that it 

 should be coasidcrcd a granite. 



At the sides of tho vein next tho slate, the n ck is of a more homogo- 



