422 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [Mar. 14, 



and north.- western part of the district, gneiss and granite appear. 

 Among the more useful subordinate rocks is a calcareous variety of 

 the mica-schist (limestone), in beds and in lenticular masses, con- 

 taining as much as 90 percent, of carbonate of lime. Another stra- 

 tified rock found in close connexion with the mica-schist is clay- 

 slate, lying on the north side of Ben Lawers, and at a great elevation. 

 The difficulty of access is the only cause why it is only rarely used 

 as roofing-slate. These subordinate strata participate in most cases 

 in the general bearing of the mica-schist, which is nearly east and 

 west ; while the dip of the mica-schist, though in general low, 

 appears as often exceptional as conformed to rule. While in special 

 cases the underlie surrounds the several axes of elevation, sloping 

 from them on all sides, both the strike and clip are not unfrequently 

 altered by the appearance of eruptive rocks and other disturbing 

 masses, as, for instance, by intersecting veins and ridges of quartz. 

 The latter mineral in general seems to have played a considerable 

 part in metamorphosing and silicifying the adjacent mica-schist, 

 having in many instances made its appearance after the formation of 

 the mica-schist. Although those disturbing causes have exercised a 

 great influence upon the original beds of the rock, as may often and 

 very clearly be observed, they nevertheless leave sufficient traces of 

 the strata having had originally a horizontal position. 



Tomnadaslian Mines. — Among the igneous rocks, porphyries and 

 greenstones deserve particular notice, on account of their frequency 

 throughout the district. As a special case, probably illustrating all 

 the others, the rocks near the middle of the south side of Loch Tay, 

 opposite Ben Lawers, at the Tomnadashan Mines, invite close inspec- 

 tion; for in that place mining-operations have extensively aided 

 observation. All the facts there obtained support the supposition, 

 that, after the mica-schist had been broken through by the green- 

 stone, a powerful vein of porphyry was erupted ; for the greenstone, 

 while remaining unchanged in position and character on the east and 

 west sides of the porphyry- vein, has near its middle part not only 

 been deranged by mechanical force, but has also been often altered by 

 chemical agency into a substance exhibiting the mixed characters of 

 greenstone and porphyry, with transitions from one to the other. It 

 is not difficult to arrive at a probable estimate of the period when 

 the mineral ingredients — silver-ore, copper-pyrites, grey copper-ore, 

 iron-pyrites, and molybdenite — have made their appearance, if the 

 porphyry be considered as the matrix ; for, while the porphyry- vein, 

 although about half a mile wide, and extending about three miles 

 towards the south, is nowhere destitute of one or other of the above- 

 named minerals (particularly iron-pyrites), the greenstone, when in 

 its original condition, nowhere contains a trace of those substances. 

 The only qiiestion remaining would be, whether those ores belong to a 

 later formation than the porphyry itself, or whether they are contem- 

 poraneous deposits. The general dissemination of these minerals 

 through the porphyry appears to prove the latter view. Closer in- 

 spection shows that the greenstone exercised a particular influence 

 upon the deposition and accumulation of the minerals named, which 



