138 Evolution and Distribution of Fishes 



invertebrate — rarely fish — organisms, the former indicates 

 largely or wholly freshwater conditions. 



But while in some localities or regions there seems to 

 be direct conformability and therefore continuity, from the 

 top of the Old Red to the base of the Calciferous or Missis- 

 sippian, in other cases a considerable period must have 

 elapsed, when terrestrial denudation and environal changes 

 were proceeding. That profound changes in the relation 

 of land and sea were also in progress is evident, and is al- 

 most certainly to be explained by the frequent volcanic out- 

 bursts, accompanied by extensive deformation and faulting 

 of strata through earth shrinkage. Regarding such Geikie 

 says: "One of the most singular features of the Lower 

 Carboniferous rocks of Scotland is the prodigious abund- 

 ance of the intercalated volcanic rocks," and he distin- 

 guishes two great types of these, ( i ) the plateaux "where 

 the volcanic materials were discharged so copiously that 

 they now form broad tablelands or ranges of hills, some- 

 times many hundreds of square miles in extent, and 1500 

 feet or more in thickness;" and (2) puys or vent-cones 

 where small and local amounts of lavas, and more copious 

 showers of ash were thrown out. Such must undoubtedly 

 have caused widespread destruction to animal and even 

 to plant life, while it gave occasion later for evolving and 

 commencing new types to multiply and spread abroad. 



The writer spent the leisure time, during years of his 

 earlier scientific career, in tracing these Calciferous rocks 

 over extensive tracts of the Forth basin, and as already 

 noted he was constantly impressed by their predominant 

 freshwater character, as well as the great thickness of the 

 strata involved. Composed of varying beds of sandstone 

 that were of a reddish-yellow or white color, of hard black 

 fissile shale, of softer argillaceous layers, of extensive fresh- 

 water limestone beds, or of brown-black bituminous oil 

 shales, these all yielded a greater or less abundance of 

 associated plant and animal remains. But as the writer 

 has already shown, in unravelling the history of Lepido- 

 phloios {g2: 181), it often happened that the bulkier por- 

 tions of some plant might be found in one set of beds, 



