S6 Evolution and Distribution of Fishes 



already named, is familiar to everyone who has dissected 

 them." 



In view of the prodigious quantities of fishes that must 

 often have been destroyed wholesale through volcanic 

 activity (pp. 120, 148) during the Old Red to Triassic per- 

 iods, it might be expected that more abundant remains of 

 these would have been preserved in the different rock- 

 strata of freshwater origin. Thus we have manifold proofs 

 that Selachians existed in freshwater areas from Devonian 

 on to Permian days at least, and in the sea also for a 

 relatively short period during Carboniferous times, but 

 abundantly from Upper Jurassic and Lower Cretaceous 

 times onward. But rather scant traces of them are met 

 with in the fossil state, unless they developed some resisting 

 parts such as teeth, spines, scales or jaws. Except for 

 these, and for the very rare and sudden preservation of 

 them in entire state in the Solenhofen, the Wyoming, and 

 other hard fine fissile rocks, we would have been unaware 

 of their existence. The soft and oily tissues as such have 

 nearly always disappeared. 



The above data all point strongly to the conclusion that 

 petroleum products are mainly due to destructive decom- 

 position of fish-oils, resulting from wholesale death of fishes 

 during different geologic periods of the earth's history. 

 Such widespread destruction seems in nearly every case to 

 be explainable by direct or indirect volcanic action, such 

 as poisonous gases, volcanic ashes or scoriae, lava flows, 

 earth and water concussions, or supramaximal heating of 

 areas in which fishes lived. But the accumulating petro- 

 leum products may have been slightly added to by simul- 

 taneous or later destructive decomposition of vegetable 

 products. Thus Redwood (25:1:47) says while describ- 

 ing the Assam oil-fields: "Mr. F. R. Mallet has enum- 

 erated the districts where oil has been noticed in the dis- 

 tricts mentioned. Thick soft sandstone is the principal 

 rock traversed by the drill, but here clay also occurs. The 

 oil always rises on or near the outcrop of the coal-bearing 

 group, usually near the outcrop of a seam or seams of 

 coal. Mr. Mallet records an instance in which the oil 

 oozes from the coal itself, though as he points out, this 



