Vol. 60.] METAMORPHISM IN THE LOCH-LOMOND DISTRICT. 27 



Lomond district, but the constructive metamorphism is very 

 different, for nothing like the albite-gneisses has been observed in 

 the Aberfeldy district. If albite be present at all, it is in small 

 water-clear granules mixed with the granulitic quartz-folia. 

 Chlorite is absent, and in its place occur folia of intergrown 

 muscovite and biotite with a considerable development of garnet. 

 We have been accustomed to consider this type of alteration as due 

 to an essentially-thermal metamorphism. I venture to suggest 

 that the albite-gneisses are due to a hydrothermal 

 type of metamorphism. The absence, or presence only in very 

 small number, of garnets : the leaching-out of the siliceous and 

 felspathic materials into separate folia ; the fact that the albitesonly 

 begin to develop after the plagioclase has been destroyed, and after 

 the removal of lime as carbonate (of which there is evidence) ; and 

 the association of the albites with a hydrated mineral, chlorite, all 

 point to this conclusion. The fact that a thermal contact at once 

 destroys the development of albite adds confirmatory evidence. 

 The view that we are dealing with a hydrothermal type of con- 

 structive metamorphism is not inconsistent with the observations 

 of Tschermak and other Continental geologists, who found that 

 albite-gneisses formed a transitional stage between slightly and 

 highly-altered sediments. 



X. Recapitulation. 



To recapitulate, we are dealing in the Loch-Lomond district 

 with a progressive metamorphism, each stage of which can be 

 accurately determined, and each process of which can be studied, as 

 a rule, without confusing its effect^ with those due to another 

 process. In the first place, we saw rocks from the Leny-Grit Group 

 and Aberf oil-Slate Group yielding evidence of dynamic metamorphism 

 not in a high degree, and of practically no constructive meta- 

 morphism whatever. Then, entering a higher stratigraphical 

 horizon, the Beinn-Ledi Group, we saw the dynamic metamorphism 

 increasing, and at Hudha Mor the beginning of a constructive 

 metamorphism of the thermal type, which was quickly superseded 

 by a constructive metamorphism probably of hydrothermal type, 

 under which, combined with, or preceded by, the increasing dynamic 

 metamorphism, the rocks rapidly became more highly crystalline 

 until all clastic structures had been obliterated. The segregation of 

 like materials into folia, the total recrystallization, and the genesis 

 of new mineral-groupings, resulted finally in the production of 

 coarsely-crystalline albite-gneisses from a series of fine and coarse 

 siliceous and felspathic grits. Finally, we have seen the effects 

 of contact with plutonic igneous masses, in the obliteration of 

 many of the results produced by the hydrothermal constructive 

 metamorphism. 



