SECTIONAL TRANSACTIONS.— 0. 555 



Taking the total collection of boulders we find that the rock groups of the above 

 tables occur in the following proportions : — 



% 



(Lavas ... ... ... ... 31 



Caledonian igneous rocks • Hypabyssal ... ... ... ... 12 



(Plutonic 8J 



Highland Border Series ... ... ... ... ... ... 15 



Other ' Highland ' Metamorphic Rocks 33 J 



Approximately half of the boulders consist of igneous rocks which are products of 

 the Caledonian magma. A very large proportion of the effusive types are not repre- 

 sented in the local Old Red Sandstone volcanic succes.sion. It is inferred that they 

 have been derived in part from a suite of volcanic rocks which formerly mantled the 

 Dalradian area. Boulders of the ' Buchan Ness Porphyry ' are so abundant as to 

 suggest the denudation of great sheets of that rock which has now a much restricted 

 distribution. The large percentage of Highland Border rocks clearly points to a 

 much wider distribution of these at early stages in the denudation of the Caledonian 

 Mountain chain. Of particular interest is the discovery of several boulders of a 

 fossiliferous limestone similar to the Margie limestone of Aberfoyle. 



The chief interest of the ' Highland ' boulders centres in the support their distribu- 

 tion gives to the idea of progressive depth metamorphism. The sillimanite and 

 staurolite gneisses of the southern highlands appear first in the highest conglomerates 

 of the Garvock group. 



Dr. D. A. Allan. — A Preliminary Account of certain Lower Old Red Sand- 

 stone Conglomerates in Perthshire and Forfar. 



The Lower Old Red Sandstone sequence along the fringe of the Grampians in 

 Perthshire and Forfarshire consists of lava flows and conglomerates with an overlying 

 group of sandstones. Owing to the scarcity of fossiliferous horizons, reliance has had 

 to be placed upon persistent groups of lavas in the mapping of the area. As the 

 general order of the lavas appeared invariable, an investigation was made of the 

 composition of the intervening beds of conglomerate which fall into five well-defined 

 groups, forming with the lavas a stratigraphical sequence. 



In the valley of the upper Ericht and the adjacent Ardle local basement con- 

 glomerates intervene between the lowest lavas and a floor of steeply folded Dalradian 

 schists. The material consists in the main of vein-quartzes, schistose grits and mica- 

 schists, together with a small quantity of igneous rocks, in part probably basic lava 

 flows and in part intrusions. 



Above the lowest lava group is a second series of conglomerates the material of 

 which is almost exclusively of igneous origin. It is in part closely similar to that of 

 some of the underlymg basic lavas, but the majority of the pebbles consist of acid 

 rocks of general lava-form character, the parent mass of which has not been identified 

 within the district. 



The third group of conglomerates is again predominantly igneous in complexion, 

 but the content of basic volcanic rocks is greater than that of the acidic varieties, and 

 there is always present metamorphic material of Highland origin. 



The fourth set of conglomerates overlies the highest group of lavas, and is composed 

 almost entirely of basic igneous rocks. Above it, but separated by a fairly persistent 

 band of sandstones, is the fifth and highest conglomerate belt, characterised by a 

 very high content of quartzites, schistose grits and vein quartzes. 



With the exception of the first, wliich is of comparatively limited distribution, 

 these conglomerate belts can be traced for long distances, and are therefore useful 

 stratigraphical indicators. The general trend of variation in their composition is 

 indicative of the denudation of a landmass of Dalradian schists which was later 

 covered by extensive sheets of igneous rock, in their turn also subjected to erosion. 

 The reversion to Highland material shown by the youngest Lower Old Red Sandstone 

 conglomerates suggests that denudation had again reached the Dalradian basement. 

 Contemporaneous erosion has been demonstrated in certain field exposures. A 

 statistical examination of the composition of the conglomerates bears this out in a 

 very striking way. While schists showing a relatively high grade of metamorphism 

 are most abundant in the lowest conglomerates, pebbles of granite, &c., do not appear 

 until much higher in the sequence. It is possible, however, that the wide distribution 

 of mica in the higher sandstones reflects the more complete destruction of such 

 Dalradian material, consonant with the greater distance it had travelled. 



