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points. They are largely hornblendic — diorite and gneiss — with 

 thin beds of a distinct mica slate ; while there is but little to 

 represent the felsitic and quartzose beds of Reading and Wo- 

 burn. This agrees with the observations made in those towns, 

 however, that the rocks become more hornblendic and micaceous 

 toward the border of the formation. The stratified rocks in 

 Marlboro' are cut by fine-grained granites, which are mica- 

 ceous, and may be Montalban. In fact, the whole band might 

 very well be referred to this system, rather than the Huronian, 

 so far as lithological characters are concerned. It is in its 

 geognostic relations, chiefly, that we find evidence justifying 

 the correlation here made. Southward from Concord, also, the 

 country is largely drift covered ; but it is probable that the 

 stratified group covers the most of the area between Concord 

 and Natick marked as general Huronian on the map. In the 

 vicinity of Snake Creek, in the eastern part of Wayland and 

 northern Natick, there are distinct gneisses, which belong, 

 lithologically, to the Montalban series ; iand it is possible that 

 the Montalban boundary is deflected this far to the east, but 

 more probable that the strata are Huronian. The strike is very 

 variable, average N.E.— S.W. ; dip, vertical, or steep to N.W. 

 These beds are cut by extravasated masses of the adjoining 

 granite and diorite. In Natick, well exposed in the railroad 

 cut east of the village, are quartzite, diorite, fine-grained horn- 

 blendic gneiss, and coarse granitoid gneiss, all distinctly and 

 conformably interstratified. These rocks cover a considerable 

 area in Natick, probably larger than I have indicated on the 

 map ; and at many points they exhibit immense disturbance 

 and alteration, the stratified diorite and hornblendic gneiss 

 becoming structureless and eruptive. As usual, the quartzite 

 is most abundant toward the south ; and it appears, as previ- 

 ously stated, to pass, on the east, into the petrosilex in Welles- 

 ley. Prof. Hitchcock mentions ' ' porphyry " (petrosilex) as 

 occurring in Natick ; but this quartzite is the nearest approach 

 to it I have observed. 



The stratified rocks in Sherborn are diorites and imperfect 



