Notices of Memoirs — Older Oranite in Lower Dee-side. 471 



probability broken down and partially reconstructed porphyritic felsites, 

 and considers that some lenticular aggregates which they contain may 

 be looked upon as broken-down porphyritic felspars. Twenty-five year? 

 ago Dr. Callaway recognized the felsitic origin of these rocks, and 

 the present investigations fully confirm that conclusion.^ Similar 

 structures have now been found in the rooks of the central area, on 

 its western coast near Trecastell. These schists, therefore, may be looked 

 upon in general as derived from acid igneous rooks. In other parts of 

 both areas, however, mica-schists in continuity with them are found in 

 intimate relations to schists of sedimentary origin, so that it is probable 

 that pyroclastic material was present in the original igneous series. 



X. — The Oldee Granite in Lower Dee-side.* By G. Barrow, F.G.S. 



A BRIEF description was given of the mode of occurrence and 

 composition of one of the older Granite intrusions in Lower Dee-side. 

 In place of forming large coherent masses it tends rather to minute 

 subdivision, permeating the crystalline gneisses over large areas. Excel- 

 lent examples of lit-par-lit intrusion may be seen on the north side 

 of the Dee, about, and west of, Banchory. The granitic material in 

 these cases forms minut(e sills, varying from an inch to several feet 

 thick, and almost rigidly parallel to the foliation of the associated gneiss 

 into which it has been intruded. The ground here is comparatively; 

 flat, and the method of feeding the sills cannot be clearly made out. 

 But on the opposite or south side of the river the ground is much 

 steeper, and in the hill-faces dyke-like intrusions can be seen, from 

 which the sills proceed. They commence a little below the crest of 

 the dyke, where they are smallest and shortest ; they are seen to 

 become steadily thicker and longer as we descend further below the 

 crest of the dyke. In the interior of the latter the granite is usually 

 grey, and contains more bioti'te than muscovite ; oligoclase is also 

 usually abundant ; the oligoclase and biotite steadily diminish in 

 amount as the rock is traced towards the taper end of the sills. At 

 this point there is little oligoclase, and often no biotite ; muscovite is 

 fairly common, often in large crystals, and the bulk of the felspar is 

 of alkaline composition. It appears that the fissures in which the 

 dykes occur were filled with igneous material, and that under great 

 pressure the walls were burst open and the still liquid material forced 

 out, and thus separated from that which had already segregated out. 

 The phenomenon may be described as magmatic differentiation intensified 

 by dynamic action. Further, the material (pegmatite) which occurs on 

 the extreme ends of the sills is often far coarser than that met with in 

 the centre of the dyke ; the distribution of coarse and fine material 

 being thus the reverse of that usually met with in small intrusions. 



This separation of the more acid and less acid material occurs in 

 connexion with every dyke and sill over the entire Dee-side area 

 between Banchory and Aberdeen ; the pegmatitic material forming 

 a fringing margin, the breadth of which varies considerably. The 

 largest of the sill-like masses occurs at Aberdeen, and part of the city 

 is built on it ; its great fringe of pegmatite veins is well displayed 

 on the banks of the Dee near the railway bridge. 



The separation of the material rich in oligoclase and biotite from 

 that richer in alkali-felspar and muscovite is not confined to each 

 separate intrusion ; it holds good for the intrusion as a whole. Well 

 to the south of the Dee, especially nearer the coast, nearly the whole of 

 the intrusions are more alkaline than those nearer the Dee ; the 



^ See Brit. Assoc. Eeport, Manchester, 1887 ; also Q.J.G.S., 1897 and 1902. 

 ^ Abstract of paper read before the British Association, Section C (Geology), 

 Dundee, September, 1912. 



