1818.] Geological Transactions, Voh. Ill and IV. 135 



for controversy to Scottish geologists. The observers, to what- 

 ever party they have attached themselves, fascinated as it were 

 by the intricate and remarkable phenomena laid bare by the 

 course of the torrent, have for the most part confined their atten- 

 tion to the bed of the stream and its immediate banks : hence 

 has arisen some misrepresentation which a more excursive 

 research would have avoided. The following is an abstract of 

 the observations made by Dr. M'Culloch. 



From the junction of the Tar'rTto the bridge of Tilt, near Blair 

 Athol, a distance of about 10 miles, the glen extends in a 

 direction about N.E. and S.W. The mountains which form the 

 north-western side consist chiefly of granite, generally of a red 

 colour, but in some places passing insensibly into a variety of a 

 grey colour, and containing crystals of hornblende. This latter 

 may, on a superficial view, be mistaken for sienite, but differs 

 from that rock in containing a large proportion of quartz, and but 

 little felspar ; it also frequently contains epidote, and numerous, 

 though minute, crystals of sphene. Insulated patches of quartz- 

 rock, schist, and limestone, interstratified with each other, may 

 be seen resting on the granite ; which beds, on the south-eastern 

 side of the glen, cover the whole surface to the entire exclusion 

 of the granite. The bed of the torrent, and its immediate banks 

 through the greater part of the space already mentioned, form 

 the line along which the granite emerges from beneath the stra- 

 tified rocks, and where the following very striking appearances 

 present themselves. 



Veins of granite traverse the schist and quartz rock and pass 

 into the accompanying limestone. These veins are occasionally 

 of large size, in which case they may often be clearly traced into 

 the main body of the granite ; often, however, they appear to 

 originate and end in the limestone, and present the aspect of 

 detached lumps and irregular processes rather than of veins. 

 Sometimes they intersect and reticulate both the schist and 

 limestone, diminishing to the tenuity of a thread or a leaf of 

 paper. Sometimes thin laminae of granite may be observed 

 arranged parallel to the beds of limestone, and following every 

 flexure and contortion which it undergoes with the most perfect 

 regularity; sometimes, again, minute points of the same red 

 6ihceous matter as constitutes the thin veins of granite occur 

 inhering in the limestone, and from their minuteness are scarcely 

 to be detected, unless where the calcareous base has been worn 

 down by the action of water, in which case, these points are 

 left protruding, and thus give a rough or echinated character to 

 the surface of the limestone, Occasionally the limestone occurs 

 apparently unstratified and in the state of crystalline marble, of 

 a white colour, partially tinged by yellow and pale green. Where 

 the marble is at the greatest distance from the granite, it differs 

 little or nothing in hardness or composition from ordinary speci- 



