GEOLOGICAL AND MINERALOGICAL NOTES : NO. 5. 13 



At the suggestion of Dr. J. E. Wolff, a comparison was 

 made between thin sections of this rock and some from 

 the Penokie Gogebic Series (Michigan and Wisconsin) of 

 Van Hise. (Am. Jour. Sci., 3d Ser., Vol. 31, 1886, p. 

 453.) The resemblance is marked, although Prof. Van 

 Hise finds the rock in limited quantities, and in a very dif- 

 ferent region, geologically. These rocks appear to belong 

 to the same series, which in the case of our rock is clearly 

 Cambrian. 



(4). Another extensive outcrop of these metamor- 

 phosed sedimentary rocks is seen in Essex, in the valley be- 

 tween White and Powder House hills and extending across 

 Essex to Conomo Point. Here the slates, which are dis- 

 tinctly interbedded with granitic gneiss and quartzites, are 

 in places filled with garnets varying from microscopic size 

 to one-fourth of an inch in diameter, thus these slates have 

 been metamorphosed into garnetiferous gneiss, a form not 

 before noticed in our Essex County rocks excepting in 

 boulders on Cape Ann and Nahant. As the two regions 

 last named are in direct line with the variations of the 

 glacial stria3 on the surface of the rocks throughout the 

 county, it may be presumed that these isolated boulders 

 are remnants of glacial material originating in this out- 

 crop in Essex. 



It seems important to call attention to these points, es- 

 pecially in regard to the first two deposits (A and B), 

 which occur in large areas on Cape Ann, for they are con- 

 founded with the hornblende-granite in the report on the 

 Geology of Cape Ann (U. S. Geol. Surv., Ninth Rep., 

 1887-88). 



Peabody Academy of Science, Aug. 13, 1892. 



