SEOTIONAL TRANSACTIONS.— C. 558 



extent. This younger' movement is chiefly block-movement of older previously 

 folded material forming ' folded mountainblocks ' in the sense of Obrouchev or ' ground- 

 folds ' in the sense of Argand. 



Among other fault-systems in the horst-region that of north-western direction is 

 far prominent. Suess has it comprised under the name of the Asiatic or Karpinskyan 

 lines. If we admit that the folded mountainblocks of the present Asiatic morphology 

 are produced by the great tangential push in relation to the southward creep of the 

 enormous crustal sheet, we have also to consider the north-western fault-system in 

 Europe as the deflected and diminished eSect of the same process. 



The unifying attribute of the Altaids is their common pre-upper-Carboniferous 

 age. But they may contain structures of various trends and of various ages. 



Afternoon. 



Excursion to Aberfoyle and Loch Chon. (Leader : Prof. J. W . 

 Gregory, F.R.S.) 



Wednesday, September 12. 



Mr. A. G. Macgregor. — Metamorphism around the Lochnagar Granite, 

 Aberdeenshire. 



These preliminary notes give some of the results of a detailed study of the Dalradian 

 Schists adjoining the margins of the ' newer ' granite and diorite of Lochnagar and 

 Glen Doll ; the work is not yet completed. 



The district is included in a Geological Survey ' one-inch ' colour- printed map 

 (Sheet 65) and described in the accompanying memoir.' In the areas under discussion 

 these publications largely represent the work of Mr. George Barrow, whose accurate 

 mapping of rock-outcrops has made ' Sheet 65 ' an invaluable asset for his successors. 



According to Mr. Barrow's views, as expressed in the Sheet 65 Memoir, the meta- 

 morphism of the Dalradian Schists is everyn-here a regional thermal phenomenon of 

 earlier date than the intrusion of the ' newer ' granite and diorite and their associated 

 dyke-suite. He states repeatedly that the ' newer ' intrusions have produced little 

 or no reconstructive effect on the rocks which they cut. I believe tliis standpoint 

 to be untenable. It should be noted that Mr. Hinxman and Mr. Cunningham Craig, 

 in the ground described by them in the Sheet 65 Memoir, attribute marked contact- 

 metamorphic effects to the intrusion of the ' newer ' granites and diorites. 



North of the River Dee, between the Slugain and Feardar Bums, Mr. Barrow has 

 shown that the Black or Dark Schist Group has a hornfels aspect and contains 

 cordierite, andalusite, sillimanite and spinel. In my opinion the evidence points to 

 the numerous local intrusions of ' newer ' granite and diorite as the cause of the 

 presence of the above-mentioned ' contact ' minerals in the metamorphosed sediments. 

 This view is greatly strengthened by the recognition of severe contact-alteration in a 

 small dyke cutting the schists. This intrusion belongs to the suite of ' newer ' dykes, 

 and its contact-metamorphism must therefore be attributed to the ' newer ' granite 

 and diorite. 



Further south, in Glen Callater, I have found contact-metamorphosed rocks, 

 originally resembling members of the suite of ' newer ' dykes, enclosed in the ' newer ' 

 granite of the edge of the Lochnagar mass. It can moreover be demonstrated that 

 the Lochnagar granite has here severely baked the Black or Dark Schist for at least 

 100 yards from its margin, and has produced considerable mineral reconstruction. 

 Cordierite is abundant, large andalusite porphj-roblasts are constantlj' present, and 

 sillimanite is sometimes found. Close to the granite margin kyanite (produced by 

 pre-'newer' granite regional or regional-thermal metamorphism) is' pseudomorphed 

 by andalusite. Shimmer aggregates around regional kyanite and staurolite have 

 been converted into cordierite. The metamorphism of the Black or Dark Schist of 

 Glen Clunie at some distance from the ' newer ' granite margin is too complex to be 

 discussed here. I believe, however, that contact-effects superposed on an earlier 

 metamorphism are not confined to the inner belt of andalusite-bearing rocks. 



' 'The Geology of the Districts of Braemar, Ballater and Glen Glova ' {Mem, 

 Oeol. Surv. Scotland), 1912. 



