﻿466 PKOF. E. W. SKEATS ON THE GNEISSES [Aug. I9IO, 



and highly schistose or gneissic dacites are observed for a few 

 hundred yards. 



These occurrences are difficult to explain simply on the hypothesis 

 of contact-met am orphism, unless we are to attribute them to the 

 alteration of a special flow or band of tuff of peculiar composition 

 and more susceptible of structural change than the normal dacite. 



An alternative view is that they are the results of dynamic 

 metamorphism. Close examination of thin sections of these rocks 

 shows that some of them exhibit signs of granulitization in the 

 porphyritic felspars and quartz, and the production of strain- 

 polarization effects in these two minerals. Further, there is a 

 very noticeable parallel orientation of the minerals and a more 

 extreme mineralogical change than occurs in the slightly schistose 

 rocks. In the gneiss the hypersthene has completely disappeared, 

 its place being taken by secondary biotite and secondary quartz. 

 Ilmenite is rarely seen, as secondary biotite has been formed from 

 it. On this view, too, the partial distribution of the gneiss can be 

 more readily accounted for, as due to differential movements within 

 the dacite mass. These must, however, have taken place before 

 the intrusion of the acid veins, since the latter commonly cut the 

 foliation planes. They may have formed before the complete 

 consolidation of the whole mass of the dacite. 



A third view is that the structures and mineral characters of the 

 gneisses may be due to a combination of dynamic and contact-meta- 

 morphism. According to this view the foliation and granulitization 

 were first impressed on the dacite after extrusion, and possibly before 

 its complete consolidation. Following this was the intrusion of the 

 granodiorite, and at a later stage the intrusion of the acid veins 

 into the already foliated rock. To contact-metamorphism may be 

 ascribed certain mineralogical and structural peculiarities, such 

 as the criss-cross arrangement of some of the clusters of secondary 

 biotite ; while the absence of marked granulitization and other 

 strain-phenomena in some of the gneissic rocks does not neces- 

 sarily imply that these structures were never present, but may 

 be explained on the assumption that these phenomena have been 

 obliterated by later recrystallization due to heat and the passage 

 through the rock of heated vapours or solutions from the margin 

 of the granodiorite. 



I am not able as yet to discuss this problem with much con- 

 fidence, but, on the present evidence, I am inclined to favour the 

 last hypothesis. It is recognized as a difficulty in the way of 

 accepting the first explanation, that practically no clear case has 

 yet been described of a gneiss having originated solely by the 

 contact-metamorphism of a volcanic rock. In the memoir by 

 Mr. Kynaston quoted above (p. 464) on the alteration of a mass of 

 porphyrite by the Ben Cruachan granite, is a description of a band 



