310 Proceedings of Philosopfilcal Societies. [April, 



Hence it becomes a matter of considerable importance that the 

 real nature of the facts appealed to by both parties should be clearly 

 ascertained. 



Most of the previous observers appear to have confined their 

 researches to particular parts in the bed of the Garry (the river 

 which flows through the Glen), where the rocks may be observed 

 with least trouble, and where the junctions of the granite with the 

 schist are the most obvious. In the present memoir the entire bed 

 of the river from one end of the Glen to the other, where it was 

 accessible, is particularly described, as well as the slopes of the 

 mountain which form the sides of the Glen. 



The right side of Glen Tilt consists for the most part of red 

 granite, passing sometimes by the intermixture of crystals horn- 

 blende into sienite. This granite may be traced to Ben Deary, and 

 thence to the great body ot granite forming the centre of the High- 

 lands, and of which body the right bank of Glen Tilt may be consi- 

 dered as the southern boundary. 



The surface of the granite is irregularly undulated and protube- 

 rant. Incumbent on the granite are several stratified rocks, the 

 lowest of which appears to be quartz rock, which is covered by, and 

 alternates with, mica-slate, either pure or passing into gneiss, with 

 clay-slate and with granular lime-stone, either the common blue 

 variety, or white rnarble, which latter sometimes contains veins and 

 concretions of noble serpentine, and is sometimes interstratitied 

 with thin beds of tremolite and of sahlite. 



The beds lying in the granite are sometimes broken off abruptly ; 

 and this latter rock making its appearance in the intervals frequently 

 gives the appearance of an alternation of granite with the stratified 

 rocks that in reality lie only on its surface. 



The stratified rocks are for the most part very regular in their 

 position ; but where they are in contact, or approach very near to 

 the granite, very remarkable disturbances, and other changes, are 

 observed to take place. The lime-stone is much indurated, and 

 more siliceous than usual. Sometimes it contains small grains of 

 reddish-brown felspar; sometimes the lamina of the lime-stone are 

 separated, and the interstices are filled up with granite ; sometimes 

 veins of various magnitude pass from the body of the granite into 

 the superincumbent beds, occasioning flexures, fractures, and 

 various intimate and fantastical intermixtures of all the adjacent 

 rocks; sometimes, on the other hand, lumps and masses of the 

 stratified rocks are to be found involved in the granite. 



Quartz rock is not always the immediate covering of the granite; 

 on the contrary, the several varieties of schist, and even lime-stone, 

 are sometimes found in that position, a circumstance that appears 

 to be irrcconcileat)le with the theory of the stratified rocks having 

 been deposited on a basis of pre-existing granite, but easily ac- 

 counted tor on the theory of the forcible irruption of the granite 

 from below in a fluid state : in which case it would rupture and 

 penetrate the superincumbent beds, more or less, according to the 



