Vol. 50.] ANJSIVERSARr ADDRESS OE THE PRESIDENT. 85 



— belonged to the superficial warm currents passing over shallow 

 plateaux, or to the tepid waters accumulated in closed basins, 

 though we may imagine that these latter might not especially favour 

 the growth of reef-corals. 



We must now consider the report of Messrs. Peach and Home on 

 the recent work of the Geological Survey in the North-west High- 

 lands, in so far as it refers to the development of beds of Cambrian 

 age within that area. It has long been known that there is a 

 marked discordance between these and the underlying formations ; 

 so that the base of the system is well-defined, and there is not 

 that obscurity about it which obtains in other areas and notably in 

 North Wales. The lowest beds of the whole series are false-bedded 

 grits and quartzites with brecciated conglomerates at the base. The 

 total thickness of the arenaceous or quartzite series is about 500 feet, 

 the upper half being distinguished as the ' Pipe-rock,' owing to the 

 number of vertical pipes of ScolitJius with which it is permeated. 

 The middle group is limited in thickness. The lower portion 

 of it, from 40 to 50 feet thick, is known as the ' Fucoid '-beds, a 

 very mixed series, often a matted network of the flattened excrement 

 of worms — an appearance misleading the old observers, who re- 

 garded them as the remains of seaweeds. Above this is the 

 curious formation known as the Serpulite-, or more correctly Salte- 

 rella-grit, about 30 feet thick, passing upwards into the ' calcareous 

 series.' Here, again, the old observers were mistaken, iSalterella 

 being a pteropod, tubular in shape and consisting of several hollow 

 cones placed one within the other. The ' calcareous ' series has a 

 thickness of 300 or 400 feet, and is divided into seven groups, all of 

 which are developed at Durness, and it is the highest but one of 

 these groups which has yielded the bulk of the fossils. In the line 

 of complex structure, south of Durness, it is only the lower, 

 generally the lowest group, which occurs ; and thus the unfossili- 

 ferous character of the Cambrian Limestones in Assynt and other 

 localities is, to a certain extent, explained. 



A brief notice of Mr. Peach's excellent summary of the physical 

 conditions of deposit and horizon of these North-west Highland 

 Cambrians seems appropriate at this stage. In the case of the basal 

 quartzites, he says, where there is a passage from a land-surface 

 to a sea-bottom, little or no organic matter was mingled with the 

 coarse siliceous sand ; consequently there was no food for the 

 support of the annelids, which became so numerous in the upper 

 beds of the quartzite. Hence the ' Pipe-rock ' indicates slower 

 vol. l. g 



