378 C. CALLAWAY ON THE NEWER GNEISSIC 



and enlarged by subsequent more deliberate study, led me to con- 

 clude that the supposed interstratifi cation of "Logan Rock" was 

 one of three things : — 



(1) The seamy- layers of the lower beds of the quartzite, greened 

 by vegetation at the outcrops. These bands, when broken into, 

 proved to be quartzose grit. 



(2) Intrusive dolerite, which in one section was clearly seen to 

 break up through lower beds, and spread itself out between quartzite 

 strata. This rock bears some resemblance to certain green horn- 

 blendic bands in the gneiss ; but it displays no signs of foliation, 

 and has the characters of an ordinary dolerite. 



(3) " Logan Rock " repeated by faults, the rock which succeeded 

 it being in every case the well-known seamy quartzite. 



I am not aware that a passage between the " Logan Rock " and 

 the adjacent formations has been alleged for any other localities. 



5. Alteration and crushing of the " Logan Mock " at junctions 

 with other formations. — After having examined junctions in some 

 scores of sections *, I cannot recollect a single case in which the 

 gneiss did not display signs of alteration at junctions. This evi- 

 dence was seen in one or more of the following particulars : — 



(1) The gneiss was green in colour, owing to the presence of a 

 chloritic mineral (No. 63, p. 416). This mineral was either dis- 

 seminated through the mass or occurred in thin folia. 



(2) Even when there was no alteration-product present, the 

 gneiss was usually soft and decayed, offering a marked contrast to 

 the typical " Logan," which is tough and sound. 



(3) The gneiss presented a confused, "messy" appearance. 

 These kinds of alteration are such as we expect to find at faulted 



junctions. Atmospheric waters, sinking down the fracture, would 

 find ample material for the formation of hydrous magnesian sili- 

 cates. The rotten state of the gneiss is accounted for by the 

 decomposition which has taken place. In the partial dedolomi- 

 tization of the magnesian limestone in contact with gneiss, already 

 described (p. 362), we have an apparent example of the chemical 

 reaction of the gneiss upon the dolomite. 



The crushing of the gneiss in contact with newer rocks is very 

 marked. This . cause, combined with decomposition, often renders 

 it impossible to obtain a clean fracture. The evidence is still clearer 

 under the microscope (JSos. 63, 73, 74, 80, 81, 83-91, pp. 416, 417). 

 A few of these specimens have a very fragmental aspect; and I 

 think it possible they may be gneissic debris, highly altered. They 

 are, however, regarded by Prof. Bonney as Hebridean crushed in 

 situ, though his opinion is not free from hesitation f. The position 

 of these perplexing rocks is always between the sound gneiss and 



* I include Ullapool in this generalization. 



J" [The sole reason for this hesitation was that the opinion was formed on 

 the slides alone, knowing nothing of their stratigraphy ; and now and then a 

 crushed rock simulates to an extraordinary extent a true sedimentary rock. 

 In several of the slides I had no doubt whatever I had " crushed Hebridean" 

 before me. -T. GL B.] 



