398 LITHOLOGICAL OBSERVATIONS 



nity. The porphyry contains crystals of horn- 

 blende and pistacite, with small portions of chlo- 

 rite, and occasionally common garnet and pyrites. 

 It also frequently affects a slaty structure. 



In the same direction, the clay-slate is succeed- 

 ed and covered by grey-wacke, and grey-wacke 

 with a slaty fracture ; both of a peculiar compo- 

 sition, and probably alternating with the older 

 rocks. This substance is an aggregate of the 

 usual fragments and materials, studded with blue 

 quartz, common quartz, and crystals of felspar. It 

 is found ascending to a considerable height above 

 the level of the water, till it is lost under the 

 heathy ground ; but it re-appears in the bed of 

 the stream above mentioned. I did not find grey- 

 wacke-slate, properly so called. In this portion 

 also, of the formation we ^re describing, the same 

 predominance of quartzy materials, Vt^hich was 

 formerly noticed, is still remarkable, not only in 

 veins and separate masses, but pervading the sub- 

 stance of the rocks. 



The islands of Loch-Lomond, are placed in the 

 line of clay- slate and transition- strata ; and con- 

 sequently, as already observed, are formed by the 

 more durable varieties of these rocks, which may 

 be traced in continuation, on the opposite shore 

 around Luss. The quarries of Luss and Cam- 

 stradden, in which the finest clay or roof slate is 

 procured, are well knov/n. They belong to the 

 great formation of clay-slate, which has beeu 



