42 



planes ; whereas in the other slates the bedding is traversed by a 

 second set of planes, those of slaty cleavage, perpendicular to the 

 bedding or stratification. 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. 



The great upward movement attendant on the elevation of the 

 mica slate mass of Benlomond, has thrown the two other slates 

 several miles southward of the line of bearing which they observe to 

 the eastward of the mountain ; and in like manner in other tracts 

 along the Highland frontier, horizontal displacements on a great 

 scale appear to have affected the two upper slate bands, so that the 

 middle or dark slate is often seen in direct continuation of the upper 

 chloritic band, or vice versd. And thus the appearance of frequent 

 alternation is produced when these bands are traced from point to 

 point on the line of bearing. 



Plutonic and Metamorphic Rocks. 



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

 which possess considerable interest. 



To the right of the summit level of the road between Gareloch- 

 head and Portincaple Ferry, a remarkable ridge projects from the 

 smooth outline of the hill side, and trends in a straight line towards 

 the base of the mountains. It consists of a highly felspathic rock 

 erupted through the slate. The base is a very compact mixture of 

 quartz and felspar, in which crystals of felspar and mica are im- 

 bedded, the latter ingredient being constantly present, the former 

 often wanting. The prevailing colour is yellowish-red, given by the 

 felspar in the base. From the prevalence of felspar, and the mode 

 in which the crystals are disseminated, the rock must be called, 

 according to the present views, a felspar porphyry ; though the con- 

 stant presence of mica as a constituent, and the compound character of 

 the base, seem rather to require that it should be considered a granite. 



At the sides of the vein next the slate, the rock is of a more 

 homogeneous character, resembling a compact claystone or flinty 

 slate, but still enclosing crystals of mica. The laminae of the slate 

 in contact with the sides of the vein have a very close resemblance 

 to this variety: being, in fact, a yellowish-gray fine grained flinty 

 slate, with occasional spangles of mica. It is thus difficult to de- 

 termine the exact boundary line between the slate and the vein. 

 This assimilation of mineral character has obviously been induced by 

 the coolinjj of the masses from a state of fusion, at nearly the same 

 rate, but jnore rapidly than the inner portions of the vein. The 



