IN THE NOETH-WEST HIGHLANDS OP SCOTLAND. 407 



whose calcareous and siliceous skeletons have slowly built up the 

 great mass of limestone and chert so conspicuously developed at 

 Durness. That small pelagic animals played the chief part in the 

 formation of this accumulation of limestone, is rendered almost 

 certain by the fact that most of the beds are traversed by worm- 

 casts in such a manner that nearly every particle must have passed 

 through the intestines of worms. It is evident from the prevalence 

 of these Annelid traces that the limestones cannot be due to coral- 

 reefs, but must be of detrital origin. Only one undoubted 

 specimen of coral, resembling a Michelinia, imbedded in a fine 

 calcareous sediment, has been obtained from the series. That 

 shell-banks had little to do with the accumulation of the limestone, 

 is apparent from the mode of occurrence of the shells which are 

 found in it. The most abundant forms are chambered shells, such as 

 Orihoceratites^ Lituites, and Nautilus ; next in order are the Gastero- 

 pods, chiefly Madurea and Pleurotomariay while the Lamellibranchs 

 and Brachiopods rank last in point of number. The two latter are 

 found with their valves attached, and the Lamellibranchs occur 

 in the position in which they lived and died. All the specimens 

 show that every open space into which the mud could gain access 

 and the worms could crawl is traversed by worm-casts. In the 

 case of the Orthoceratites, they seem to have lain long enough un- 

 covered by sediment to allow the septa to be dissolved away from 

 the siphuncles which they held in place. Many of those siphuncles 

 are now found isolated ; indeed Salter established his genus Filoceras 

 on such large examples as those found in Endoceras. Sponges of the 

 genera Archceocyathus and Calathium occur at intervals in the muddy 

 matrix. One example is preserved in chert ; but the larger masses 

 of chert in the limestone do not seem to be derived from sponges, 

 but more probably from the siliceous skeletons of Diatoms, which, in 

 all likelihood, were as abundant in that ancient ocean as they are 

 now. No undoubted remains of Poraminifera have been discovered, 

 though on several horizons there are zones of limestone made up of 

 small rounded bodies, probably oolites ; but owing to the fact that 

 the limestones are crystalline, and that many of them have been 

 more or less dolomitized, it is now almost impossible to decide defi- 

 nitely as to the nature of these spherules. Por the same reasons it 

 is almost hopeless to find minute organisms in this formation. The 

 shell-substance of the larger fossils has in almost every case been 

 dissolved out, and the spaces have been filled with calcite and, in 

 some cases, with beekite, so that all the finer markings on their sur- 

 faces are obliterated. 



The fossils, as Salter long ago pointed out, are distinctly of an 

 American type, and do not resemble those found in the contempo- 

 raneous deposits of Wales and England. So far as the order of suc- 

 cession of the beds and their fossil contents are concerned, we have 

 almost an exact counterpart of the strata exposed along the axis of 

 older Palaeozoic rocks, stretching from Canada through the eastern 

 States of North America. In the latter region the Silurian strata 

 of Sutherlandshire are represented by : — (1) the Potsdam Sandstone, 



