In Silurian and Devonian Epochs 109 



pointed out, in the above quoted work, the curious facts 

 that living Cyclostomes are rich in oil, and also that sup- 

 plies of petroleum or rock oil, as well as abundant remains 

 of conodonts, first appear synchronously in Cambrian and 

 Silurian rocks. It may be hoped that added evidence will 

 soon be obtained as to the possible affinity or not of the two 

 kinds of structure. 



In trying to condense and interpret the knowledge now 

 presented the writer would infer that most, if not all, of 

 the Lower Silurian rocks and even of the Llandovery- 

 Wenlock in England, or the Niagaran-lower Salina in 

 America, were deposited as marine accumulations, and so 

 include typical assemblages of littoral marine fossils, such 

 as are catalogued by many palaeontologists. But no un- 

 doubted fish remains occur with these. Such deposits were 

 adjacent to extensive though low-lying land-masses, which, 

 in the Upper Silurian period, were often extensively flooded 

 by invasions of the sea, so that the freshwater and the 

 marine life at times became mixed. The types at that time 

 ablest to survive such oscillatory changes, were mainly 

 species of Lingula or Chonetes, which may be found along 

 with a varied littoral marine life, or at times occurring in 

 depauperate state, or in the Upper or freshwater beds their 

 remains occur as washed out and redeposited shells, along 

 with what evidently was a widespread freshwater fauna. 



Such oscillatory changes were due to widespread vol- 

 canic action, as recorded by various geologists, and as sum- 

 med up for East Scotland by Campbell as follows (op. cit. 

 935) • "One must conclude that early in Downtonian times 

 (or perhaps in pre-Downtonian but subsequent to the move- 

 ments which folded the eastern schists) volcanic activity 

 had already begun in the schist country to the north of the 

 Highland Fault." 



But In pre-Downtonian and Mid-Salina times the sea 

 gradually retreated and left extensive N. European — and 

 evidently N. E. American — continental areas of no great 

 elevation, that were traversed by far-reaching river systems. 

 These rivers must have often expanded into marshes or 

 shallow lakes, that in their periodic change of level during 

 dry and wet seasons, simulated the drainage areas of the 



