388 REPORT OF NATIONAL MUSEUM, 1904. 



relied upon to give a clew to their origin, and it was riot until the thin 



sections made it possible to sometimes trace a distinct gradation from 



schistose and foliated rocks of decidedly metamorphic 



Dana's Views on . . . , " 



Metamorpnism, aspect into massive eruptives ot the same ultimate com- 

 position that a feeling of doubt began to arise in the 

 minds of observers regarding the assumed sedimentary origin of the 

 gneisses and crystalline schists. 



It was during this prematinal period, as Rogers might have termed 

 it, that J. D. Dana came out with a suggestive paper in the American 

 Journal of Science on the analogies between the modern igneous rocks 

 and the so-called Primary formations and the metamorphic changes 

 produced by heat in the associated sedimentary deposits. The con- 

 clusions arrived at were based upon observations made during the 

 Wilkes exploring expedition, and the exciting cause of a paper at this 

 time would appear to have been the somewhat varying views recently 

 put forth by Lyell. 



Dana argued that: (1) The schistose structure of gneiss and mica 

 slate was not necessarily an evidence of sedimentary origin; (2) that 

 some granites having no trace of a schistose structure may have had a 

 sedimentary origin; and (3) that the heat producing metamorphism 

 was not applied from beneath by conduction, but was rather due to 

 heated waters of the ocean which permeated the rocks. As confirma- 

 tory of the first he called attention to the parallel arrangement of the 

 minerals and the consequent platy structure, with a tendency to split 

 along certain lines, which were sometimes found in volcanic and other 

 igneous rocks. The possibility of a schistose structure due to dynamic 

 (shearing) causes was not, however, suggested. 



His argument for the possible metamorphic origin of certain gran- 

 ites was based upon the fact that some of the basaltic tuffs observed 

 by him in the Andes and in Oregon had become so indurated that their 

 fragmental origin was almost wholly obscured. He felt that if rocks 

 of this type could be so remodeled or rehardened as to be scarcely dis- 

 tinguishable from the parent rock, sedimentary deposits of granitic 

 origin might undergo a like change. 



His argument against metamorphism by dry heat was enforced by 

 calling attention to the low conducting power of stone, which is such 

 that heat, even to the point of fusion, may be transmitted only a few 

 inches. "Lavas may be heated to a red heat within a }^ard of the sur- 

 face and still be so cool above that the bare foot may walk upon them." 

 lie believed that subterranean waters or water on sea bottoms might 

 become so heated by " volcanic tires"' as, on permeating the rocks, to 

 bring about the metamorphism. This, of course, would mean that the 

 gneissic rocks were not necessarily deep seated (hypogene), as held by 

 Lyell, but analogous to other rock formations deposited and solidified 

 at or near the surface. 



