BROOK TROUT 



into the sea, notably the salmons and the sea trout, 

 which there became habilitated. 



The effect of the ice-blanket which so long over- 

 laid the earth was to cool the earth's heated and plastic 

 crust, thereby causing shrinkage, which in turn created 

 fissures, both superficial and subterrene; and these 

 served as conduits for the fluvial output from the dis- 

 solving glacial sheet, and as passageways for the 

 salmonidse. 



'Distribution. 



Throughout subsequent ages subterranean streams 

 have played a very important part in fish distribution, 

 especially in affording passage under mountain eleva- 

 tions and high table-lands, which would otherwise 

 have proved insurmountable obstacles to superficial 

 transit. Of the existence of a subterranean fluvial 

 system co-extensive with the continental area, and 

 having intimate connection with the ocean, we have 

 abundant manifestations. The remarkable fresh-water 

 ebullitions in the Gulf of Mexico, the fathomless sink- 

 holes and mammoth springs in the adjacent peninsula 

 of Florida, the re-entering " zanates " on the coast of 

 Mexico, and the copious gushes of oil, gas, and water 

 which break out of the sands and rocks all over the 

 continent, are striking and familiar attestations. So, 

 also, sudden changes are occasionally observed in the 

 quality and color of interior ponds, with metamor- 

 phoses of their bottom floors, and of the living organ- 



