4 Proceedings of the Royal Physical Society. 



limestone zones are preserved only at Durness ; elsewhere, 

 along the line extending from Eribol to Sleat, in Skye, the 

 limestones occur in lenticular masses, which are not due to 

 original irregularity of deposit, but to earth movements which 

 gave rise to reversed faults at low angles to the horizon. By 

 this means the underlying gneiss and Cambrian rocks are 

 driven over the Silurian strata for great distances. In those 

 portions of the Silurian series which w^ere not displaced, as 

 well as in those which were driven horizontally forward, 

 there is a striking regularity in the order of succession. All 

 the zones already mentioned, including minor subdivisions not 

 exceeding a few inches in thickness, are traceable over the 

 whole area for a distance of ninety miles from north to south. 



What, then, are the inferences that may naturally be 

 deduced from a consideration of these data ? In the case of 

 the basal quartzites, where we have a passage from a land 

 surface to a sea bed, there is little or no organic matter 

 mixed with the coarse siliceous sand, which, from its coarse 

 texture and the false bedding of the layers, bears evidence of 

 rapid accumulation. There would therefore be no food for 

 the support of annelides under these conditions. But with 

 the slower accumulation of sediment indicated by the " pipe- 

 rock," there was evidently time for the fertilisation 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." It is quite consistent wdth this hypothesis that 

 the " pipes " generally become more numerous towards the 

 top of the quartzites, where some of the beds are crowded 

 with worm casts. As the sea-floor gradually subsided, the 

 shore-line was removed further from the area of deposit ; and 

 hence, during the deposition of the fucoid beds, only the 

 finest sediment derived from the land was mingled with the 

 calcareous and organic matter. 



Different species of errant annelides make their appearance 

 in the fucoid beds along with the survivors of those that 

 formed the vertical burrows in the quartzite; indeed, the 

 surfaces of the beds present a matted and entwined network 

 of their flattened excrements. These remarkable appearances 



