IN THE NORTH-WEST HIGHLANDS OP SCOTLAND. 



405 



Vertical Section I. (continued). 



A. Arenaceous 

 Sbuies. 



Upper Zone. 



Lower Zone. 



I 



/^Fine-grained quartzites, perforated by 

 vertical worm-casts and burrows, 

 becoming more nmnerous towards 

 the top of the zone ("pipe-rock" of 



i^ previous authors). 



/^False-bedded flaggy grits and quartzites, 

 composed of grains of quartz and 

 felspar. At the base there ia a thin 

 brecciated conglomerate, varying from 

 a few inches to a few feet in thickness, 

 containing pebbles of the underlying 

 rocks, chiefly of quartz and orthoclase, 

 the largest measuring about 1 inch 

 across. 



The highest beds which occur along the line of complicated structur 

 belong to the Sailrahor Limestones (Group III. vert. Section I.). 

 These dark mottled limestones, representing only the basal beds of 

 this group, occur on the limestone-plateau of Inchnadamff, and 

 nowhere else along the line. None of the rich, fossiliferous zones 

 of Durness is met with anywhere between Eriboll and Ullapool, 

 because they all occupy higher horizons. The Orthoceras found by- 

 Mr. Charles Peach in Assyut must have been obtained from one or 

 other of the bands of Serpulite -limestone at the base of the Ghradaidh 

 Group. 



3. Physical Conditions during Deposition of Silurian Strata, and 

 Horizon of the latter. 



Mr. B. N. Peach thus summarizes the physical conditions 

 indicated by the Silurian strata in Sutherland. In the case of the 

 basal quartzites, where there is a passage from a land-surface to a 

 sea-bottom, there is little or no organic matter mixed with the coarse 

 siliceous sand, which, from its texture and the false -bedding of the 

 layers, bears evidence of rapid accumulation. There would there- 

 fore be no food for the support of Annelides under these conditions. 

 But with the slower accumulation indicated by the " pipe-rock " 

 there was evidently time for the fertilization of the sand by the 

 shower of minute pelagic organisms which is ever falling on the 

 sea-floor, so that it could afford food for the burrowing Annelides 

 whose casts now form the stony pipes. 



Different species of errant Annelides make their appearance in the 

 " Fucoid-beds " along with the survivors of those that formed the ver- 

 tical burrows in the quartzite, the surfaces of the beds presenting a 

 matted network of their flattened excrements, thus misleading the 

 older observers, who regarded them as the remains of seaweeds. The 

 zone of Serpulite-grit indicates a shallowing of the area of deposit 

 and the introduction of coarser sediment ; but after its deposition 

 hardly any sediment derived from the land entered into the compo- 

 sition of the overlying limestones. Eventually nothing seems to 

 have fallen on the sea-floor but the remains of minute organisms, 



