166 A CENTURY OF SCIENCE 



synclinals. This great law of mountain structure would alone 

 lead us to suppose that the gneiss of the Green mountains, 

 instead of being at the base, is really at the summit of the series. 



We cannot here stop to discuss Mr. Marcou 's remark about 

 'the unstratified and oldest crystalline rocks of the White 

 mountains' which he places beneath the lower Taconic series. 

 Mr. Lesley has shown that these granites are stratified, and with 

 Mr. Hunt, regards them as of Devonian Age. (This Journal, 

 vol. 31, p. 403.) Mr. Marcou has come among us with notions 

 of mountains upheaved by intrusive granites, and similar anti- 

 quated traditions, now, happily for science, well nigh forgotten. ' ' 



It is seen that Marcou, notwithstanding the general 

 character of his work, happened to be nearer right in 

 some matters than were his critics, and that "T" had 

 adopted to the limit the views of Hunt. 



The recovery of geology from this period of confusion 

 was partly owing to the slow accumulation of opposed 

 facts; especially to a recognition of the fact that the 

 overplaced relation of the granite gneisses in western 

 Scotland was due to great overthrusts ; also to the evi- 

 dence of the clearly intrusive nature of many of the 

 Cordilleran granites. The recovery of a sounder theory 

 was hastened, however, by the application of criticisms 

 by J. D. Dana in the Journal. In 1866 (42, 252) Dana 

 pointed out that sedimentary rocks in Pennsylvania, in 

 Nova Scotia, and other regions which had been buried to 

 a depth of at least 16,000 feet are not metamorphic. 

 Mere depth of burial of sediments was not sufficient 

 therefore to produce metamorphism and aqueo-igneous 

 fusion. The baseless and speculative character of the 

 use of minerals as an index of age and of Hunt's inter- 

 pretation of New England geology in general was shown 

 by Dana in 1872 (3, 91). The following year Dana 

 pointed out clearly that igneous eruptions in general 

 have been derived from a deep-seated source and did not 

 come from the aqueo-igneous fusion of sediments. As to 

 gradations between true igneous rocks and fused and 

 displaced sediments he makes the following statements 

 (6, 114, 1873) : 



" Again, the plastic rock-material that may be derived from 

 the fusion or semifusion of the supercrust, (that is, of rocks 



