THE PALEONTOLOGIC RECORD 599 



de sacs), or when of small areal extent, as the result of fillings of holes 

 in the sea bottom. In all such places there is defective circulation and 

 lack of ox}-gen resulting in foul asphixiating bottoms. 



These are the " halistas " of Walther and the " dead grounds " of 

 Johnstone. To-day such are the Black Sea and the Bay of Kiel, where 

 sulphur bacteria abound in greatest profusion. These decompose the 

 dead organisms that rain from the photic region into such suffocating 

 areas, or the carcasses which are drawn there by the slow undertow from 

 the higher ground. These bacteria in the transforming process deposit 

 in their cells sulphur that ultimately combines with the iron that is pres- 

 ent and replaces the calcareous skeletons of invertebrates by iron pyrite 

 or marcasite. In this way are formed the wonderfully interesting 

 pseudomorphs of Triarthrus becM, the Utica trilobite preserving the 

 entire ventral limbs, and of the other well preserved but small inverte- 

 brates from the Coal Measures black shale of Danville, Illinois. 



Brackish- water and especially deep-sea shelled animals tend to have 

 thin shells, while increase of salinity tends towards the thickening and 

 roughening of the calcareous shells. It is a well known fact that in the 

 dolomite-depositing continental seas like that of the Guelph (Siluric). 

 all of the molluscs have ponderous thick shells. These have been in- 

 terpreted as reef-living species but actual reefs in the Guelph are un- 

 known. The molluscs are often common but corals are represented by 

 but a few species. Similar conditions are known to occur in other 

 dolomite faunas. Further, the Guelph was of a time of decided progres- 

 sive emergence and restrictional seas under an arid climate, and there- 

 fore the waters must have been abnormally salty. 



Rivers constantly discharge into the sea great quantities of plant 

 material, but as a rule little of it other than the wood is swept far out 

 to sea. At present the rivers of northern Siberia float into the sea 

 vast numbers of logs that drift with the currents to Spitzbergen, East 

 and West Greenland and Arctic America. This wide dispersal of 

 wood hj the sea is met with only in the cold regions, whereas in trop- 

 ical waters the wood is rapidly decomposed. Single leaves are rarely 

 transported far from their place of origin, and when of good preserva- 

 tion in geologic deposits, give decisive evidence of the nearness of the 

 shore. On the other hand, tough palm leaves have been seen in the 

 sea 70 miles from land and rafts of leaves are often met with 200 or 

 more miles beyond the mouths of the Kongo and the Amazon. Prox- 

 imity to shore is also indicated by the presence in marine faunas of 

 land molluscs, insects and bones of land vertebrates. 



With tillites now known in the Lower Huronian of Canada, in the 

 Lower Cambric of northern Norway, China, South Africa and Aus- 

 tralia, and in the Permic of India, South Africa, Australia and Brazil, 

 we observe the recurrence of glacial climates. The Siluric and Devonic 



