Barrel! — Growth of Knowledge of Earth Structure. 145 



gross errors. In support of his views he read in Novem- 

 ber, 1861, a paper on the Taconic and Lower Silurian 

 Rocks of Vermont and Canada. In the following year 

 he was severely reviewed bv "T," who states positively 

 in controverting Marcou (33, 282, 283, 1862) that "the 

 granites (of the Green Mountains) are evidently strata 

 altered in place." 



"Mr. Marcou should further be iuformed that the granites 

 of the Alpine summits, instead of being, as was once supposed, 

 eruptive rocks, are now known to be altered strata of newer 

 Secondary and Tertiary age. A simple structure holds good in 

 the British Islands, where as Sir Roderick Murchison has shown 

 in his recent Geological map of Scotland, Ben Nevis and Ben 

 Lawers are found to be composed of higher strata, lying in 

 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 



