468 Dr. Mac Culloch on Quarts Rock. 



summits of Jura. There is, in fact, no essential difference between 

 them, as they consist in both cases of highly compacted grains of 

 quartz, with interspersed grains of felspar, often earthy, and never, 

 I believe, so perfectly crystallized as they are in granites and por- 

 phyries, but having the aspect of fragments rather than of crystals. 

 I did not meet with any specimens on Schihallien of a formation so 

 decidedly mechanical as those which I have described as occurring in 

 Jura, but must rest the proofs I am desirous to bring forward of the 

 mechanical and therefore secondary structure of this rock, on other 

 and more circumstantial evidence. Possibly future observers, bearing 

 this doubt in their minds, may with more time and opportunity dis- 

 cover such positive proofs of this nature as I failed to find, since 

 our acquisitions in specimens are generally in proportion to our 

 previous knowledge. 



It will doubtless be admitted that if of two rocks precisely similar 

 in the generality of hand specimens, similar also in structure and 

 position, and possessing the same characters in general aspect and 

 disintegration, the one is proved to be a mechanical deposit by its 

 containing rounded pebbles or beds of breccia, it affords a very 

 strong presumption that the other is of the same nature. But we 

 have yet to see what further evidence of this may be found in the 

 neighbouring rocks, as the occurrence of a single bed of a recom- 

 posed rock in a situation so remarkable as the ridge of Schihallien, 

 might excite rational doubts, unless we could discover a greater 

 extent of similar rocks connected vv'ith it. It is here necessary to 

 remark that the whole country to the north of this ridge consists for 

 a considerable space of rocks of the class called primitive. The 

 vale of Tumel and the hills which bound it are composed chiefly of 

 mica slate, which continues till it meets the granite of the central 

 Grampians. The case is different on the southern side of Schihallien.. 



