GEOLOGY. 109 



beds of impure dark-colored ferriferous limestone, holding a 

 greenish mineral, designated as serpentine or chlorite. A sec- 

 tion from one of the more silicious beds showed by micro- 

 scopic examination quartz and orthoclase interwoven as in 

 a graphic granite, together with a plagioclase feldspar and 

 chlorite. The rocks of the Pebidian series, which, with a 

 northeast strike, rest unconformably upon the Dimetian, are 

 usually nearly vertical and sometimes inverted in attitude. 

 The portions exposed from beneath the overlapping Cam- 

 brian strata show a thickness of about 3000 feet. Besides 

 compact conglomerates, the materials of which are apparently 

 derived from the ancient Dimetian series, the Pebidian rocks 

 are somewhat vaguely described as consisting of stratified 

 porcellanites, alternating with greenish and purplish schists. 

 They are cut by dikes, which do not traverse the overlying 

 Cambrian. Besides the locality at St. David's, Hicks has rec- 

 ognized several areas of Pebidian strata in that region, which 

 have been heretofore mapped as altered Cambrian strata, 

 and others which have been regarded as intrusive rocks. 



ROCKS OF THE ARDENNES. 



The crystalline rocks of the Ardennes have been the sub- 

 ject of microscopic study by De la Vallee-Poussin and Renard, 

 according to whom many rocks hitherto regarded as exotic 

 or eruptive, including hornblendic, euritic, and porphyritic 

 masses, are reallv indigenous rocks, interstratified with the 

 schists and quartzites of the region. They infer that these 

 rocks were of aqueous origin, and became crystalline soon 

 after their deposition. 



GEOLOGY OF VERMONT. 



J. D. Dana has called attention to the observations of the 

 late Mr. Wing on the geology of Vermont. The facts are, 

 however, for the most part, not new, and Mr. Wing's obser- 

 vations were examined and discussed by Billings and the 

 present writer in the American Journal of Science in 1868. 

 It has lomx been known that the folded strata along the west- 

 ern base of the Green Mountains included fossiliferous rocks 

 of Trenton age, both in New York and Vermont, and in the 

 province of Quebec. In the latter region, strata supposed by 

 Logan to be at the base of the Quebec group, and to under- 



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