Notices of Memoirs — Volcanic Rocks, Aberdeenshire. 561 



I. — The Volcanic Eocks around the Ord Hill of Ehynie, Aberdeen- 

 shire.' By William Mackie, M.A., M.D. 



rPHESE rocks occupy an area of two-thirds of a mile in length by 

 JL a quarter of a mile in breadth, of which Ord Hill marks the 

 centre, about half a mile west of the village of Muir of Ehynie, West 

 Aberdeenshire. The group embraces at least three independent lava 

 flows, with associated tufEs and interbedded and overlying sedimentary 

 rocks, and lies on an eroded, eastward-sloping surface of the diorites and 

 gabbros of West Aberdeenshire — rocks which have been considered to 

 represent an early and basic modification of the younger Grampian 

 granite — and are cut off on the east by the boundary fault of the Old 

 Red Sandstone outlier of Ehynie and Kildrummy. A small outlier 

 limited to two or three square yards of surface, and representing) 

 a single lava-flow, lies on the edge of the serpentine mass of Cnoo 

 Cailliche about a mile south from the extreme southern end of the main 

 area. The volcanic members of the group consist of grey to greenish 

 coloured rhyolites, showing in places fine vesiculation and beautiful 

 fluidal bandinig. The usual minerals of such rocks — quartz, orthoclase, 

 muscovite, biotite with at times an occasional augite, are present in an 

 amorphous base. Flow-brecciation is a frequent feature, and evidence, 

 of the effect of pressure is not entirely absent. The tuffs consist of 

 fragments of rhyolite rocks— occasionally up to 2 inches in diameter — 

 often rounded and encircled with glassy coronas exhibiting fluidal 

 banding. Broken crystals of quartz and orthoclase, as well as con- 

 siderable fragments of older tuffs, are also present. The sedimentary 

 rocks of the group consist of very hard siliceous grits which, on 

 microscopic examination, often show the presence of fragments of 

 volcanic rocks of the same general characters as the associated lavas. 

 These fragments are generally larger in size than the accompanying 

 fragments and grains of sedimentary origin. The whole of the group 

 is characterized by the presence of numerous, often very fine, secondary 

 quartz infiltrated veins, and at one part in the sedimentary division these 

 veins make up quite half the bulk of the rock. 



Two small masses of biotite granite containing much microcline break 

 through the diorite immediately to the west of the area. With these 

 the volcanic members of the group may or may not have a genetic 

 relationship. 



As regards the age of these rocks, it is impossible to arrive at a very 

 definite conclusion. They are manifestly younger than the diorite, but 

 are probably considerably older than the oldest beds of the Old Red 

 Sandstone of the adjoining area. The lavas being throughout of acid 

 type, it is evident that they cannot be correlated with the interbedded 

 andesites of the local rocks of Old Eed Sandstone age. 



n. — The Millstone Grit of Yorkshire : Some New Evidence as to 

 ITS Source of Origin.' By Albert Gilligan, B.Sc. 



MOEE than fifty years ago Dr. H. C. Sorby attempted to trace the 

 source whence the material which makes up the Millstone Grit 

 had been derived, by making a collection of pebbles which occur so 

 abundantly in some of the beds. Among these he found some small 

 fragments of mica-schist, quartz-schist, and a few pebbles of undoubted 

 granite. The largest pebble he obtained was about 4 inches in circum- 

 ference, and of a type resembling a fine-grained syenite of greenstone, 



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

 Dundee, September, 1912. 



decade v. — VOL. IX. — NO. XII. 36 



