L20 7'. .V. Dale— Algonkian- Cambrian Boundary. 



A.m. XIII. 

 flu Green 

 Da i.e. 



I'lf Algonkian- Cambrian Boundary East of 

 Mountain Axis in Vermont,'* by T. Nelson 



During parts of three summers, 19L3-1915, the writer was 

 engaged in tracing the boundary between the pre-Cambrian 



rocks Of the Green Mountain range in Vermont and the Cam- 

 brian beds east of them. The work began on the south near 

 Heartwellville, in Keadsboro township, Bennington County 

 (lat. 42° 50' ; long. 73°), and ended on the north at the southern 

 line of the town of Stockbridge in Windsor County (lat. 43° 

 42' 30" ; long. 72° 49'), the whole latitudinal distance being a 

 little more than 60 miles, of which, however, two stretches were 

 not studied, reducing the distance to 46- 7 miles. In this the 

 boundary, measured along its meanders and sinuosities, is 57 

 miles long. 



As many years will probably elapse before the geological 

 mapping of this region is completed and the U. S. G. S. folios 

 of it reach the public, the more important results of these ex- 

 plorations are here briefly outlined. These results concern the 

 structure of the rocks and their origin and composition. 



The recent geological literature of the region consists of two 

 papers bv C. L. Whittle and an abstract of a note by Arthur 

 Keith.f " 



The gist of Mr. Whittle's papers is " that immediately below 

 the Lower Cambrian quartzite in Vermont there is a series of 

 more or less metamorphosed clastic rocks of no inconsiderable 

 thickness ; the upper member of this series being a dark 

 chloritic mica schist ; the lower member a highly metamor- 

 phosed conglomerate and between these several pebbly lime- 

 stones and pebbly micaceous quartzite strata. . . . These rocks 

 are referred to the Algonkian Period." Below the above series 

 " a still older more metamorphosed and more variable series of 

 stratified rocks of Algonkian age occurs, together with gneisses 

 and schists whose origin is unknown, and abundant metamorphic 

 equivalents of old basic rocks." 



In his article in this Journal (p. 351) he thus refers to two 

 divergent foliations. "Structurally we have stronger evidence 

 furnished by a conglomerate gneiss at North Sherburne where 

 an anti-clinal axis trending about 25° west of north represents 



* Published by permission of the Director of the U. S Geol. Survey. 



\ Whittle, Ch. L., The occurrence of Algonkian rocks in Vermont and the 

 evidence for their subdivision, Journ. of Geol., vol. ii, pp. 396-429, 1894. 



Whittle, Ch. L., The general structure of the main axis of the Green 

 Mountains, this Journal, 3d ser., vol. xlvii, pp. 347-355, 1894. 



Keith, Arthur, A pre-Cambriau unconformity in Vermont, (Abstract) 

 Geol. Soc. Am. Bull., vol. xxv, No. 1, pp. 39-40, Meh. 30, 1914. 



