Topography of the Graywacke. 



269 



in that place, belongs to this formation. That it is not the classical 

 graywacke, I am certain : nor do I feel satisfied what are the ingre- 

 dients that compose it ; though I suspect the presence of compact 

 feldspar in considerable quantity. Its dip and direction coinciding 

 with those of the graywacke, I was led to suspect that it might be 

 one of the anomalous varieties of that rock. But I shall place the 

 specimens in the collection among those from the gneiss formation ; 

 hoping that some geologist will be able to examine this rock care- 

 fully in its native situation. This I had not time to do when I passed 

 through the place. And I think it better that it should not be no- 

 ticed on the map, than be referred to any formation with no more evi- 

 dence of its true nature than I at present possess. 



It appears then, that all of the graywacke formation lying south of 

 the Blue Hills, in Massachusetts and Rhode Island, forms but a sin- 

 gle tract. The slaty varieties predominate on the west side of this 

 tract ; and on some of the islands in Narraganset Bay, this slate ap- 

 pears to be passing into the primary schists, particularly into mica slate. 

 The red quartz rock is most abundant in the south part of Wren- 

 tham near the coal mine, and in that branch of the deposite, which 

 occupies the north part of Randolph. It occurs, also, in the west 

 part of Walpole,- as already noticed, and likewise in Abington. And 

 from the bowlders of this rock which I noticed in Scituate, mixed 

 with the granite, I am suspicious that it may be found in place from 

 Abington to the coast, in a northeast direction ; though I searched 

 for it in vain. 



Around Boston we find another tract of the graywacke, occupying 

 a basin, of which the Blue Hills form a southern boundary ; the 

 porphyry hills of Lynn and Maiden a northern, and the greenstone 

 ranges of Weston and Waltham, a western boundary. The argil- 

 laceous slate connected with the graywacke, is all found along the 

 northern and southern sides of this basin, as may be seen by the 

 map. The central parts are occupied by conglomerates and gray- 

 wacke slate. 



I have already suggested the probability that Boston Harbor was 

 produced by the wearing away of the graywacke formation. That 

 this series of rock once occupied the harbor, is obvious from the 

 character of the islands, which are evidently the remnants of a once 

 continuous formation. It is true that these islands are for the most 

 part covered with diluvium : but sometimes on their shores, we find 

 rocks in situ ; and in such case I have regarded the whole island as 



