Dr. M*=Culloch's Additional Remarks on Glen Tilt. 65 



and different varieties of hornblende schist. It is probable therefore that the 

 observations of Professor Playfair and Lord Webb Seymour have differed 

 from mine, solely from the circumstance of our having examined different 

 points on the same ground. 



Marl. — I find, on examining the Perthshire agricultural report, that the 

 marl described in my original memoir (page 316 &c.) has been observed in 

 other situations. It has merely however been looked on with the eye of an 

 agriculturist, and apparently confounded with those marl beds found on flat 

 ground under peat, which are the produce of subaquatic shell-fish, and indi- 

 cate the places where small lakes have formerly existed. 



Quartz. — Besides the varieties formerly described, I have found hyaline 

 quartz in veins, in the micaceous schist of Glen Tilt : a circumstance by no 

 means common, as the quartz of veins is almost always opaque. The speci- 

 mens in question are sometimes colourless, at others of a pale smoky brown, 

 resembling the specimens that accompany the titanite in the chlorite schists 

 of Killin. I may also add, that groups of crystals of opake quartz, of a very 

 large size, occur among the granite of the surrounding mountains; a circum- 

 stance deserving of notice, only because of its rarity throughout Scotland ; 

 where, unlike to Cornwall, the quartz is rarely found in a crystallized state. 



PiNiTE. — Besides the brown porphyry in which this mineral is found, it 

 oecurs in two other varieties, the one of a pale brownish pink colour, the 

 other of a pale gray. In both these rocks it is more abundant than in the 

 one formerly described ; while, from the contrast of colour, it is at the same 

 time more conspicuous. I may add that the porphyries of these hills are often 

 of a very compound structure, containing, besides their characteristic ingredi- 

 ent, crystals of quartz, mica, and hornblende. 



Pyrites. — Occasionally small irregular nodules of copper pyrites are found 

 in the quartz veins that traverse the schistose rocks. Iron pyrites is also 

 found crystallized in cubes which are sometimes an inch in size. It seems 

 to occur indiscriminately in all the rocks; in the granite, the schists, and the 

 limestone ; but it is most frequent in the micaceous and hornblende schists. 



Sphene. — Although this mineral is of most frequent occurrence among the 

 granites that contain hornblende, it is not limited to those varieties. I may 

 also remark that the magnitude of its crystals, in the specimens in which it 



VOL. VI. K 



