President's Address. 5 



misled the older observers, and made them regard these casts 

 as the remains of seaweeds. Similar phenomena have been 

 met with in strata of the same general age in America, where 

 they have been usually attributed to the remains of plants, 

 each variety being designated by a different name. Were we 

 to proceed on this principle with the worm casts of the 

 Durness rocks, a lengthy treatise might be devoted to the 

 description of the various forms which they assume. Had 

 the matrix of the limestone been more favourable for pre- 

 serving small organisms, I have no doubt we should have 

 been able to obtain as rich a variety of annelide jaws from 

 the Durness strata as Mr Hinde procured from the nearly 

 unaltered Silurian beds of Gothland. 



The zone of serpulite grit overlying the fucoid beds points 

 to a shallowing of the area of deposit, and the introduction 

 of coarser sediment, thus indicating oscillations in the down- 

 ward movements, though on the whole they resulted in sub- 

 sidence. After the deposition of the serpulite grit, hardly 

 any sediment derived from the land entered into the compo- 

 sition of the overlying limestones. For a short time there 

 was a slight admixture of sand and mud with the calcareous 

 matter ; but eventually nothing seems to have fallen on the 

 sea-floor but the remains of minute organisms, whose cal- 

 careous 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 that the limestones cannot be due to coral reefs, 

 for if such had been the case, the corals must have been 

 comminuted as fast as they grew. Only one undoubted 

 specimen of coral resembling a Michelinea, embedded 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 Orthoceratites, Lituites, and 



