EASTERN MASSACHUSETTS. 



433 



At this localityj and elsewhere about the harbor, the syenite can be 

 seen cutting irregularly through the schist in every direction, thus form- 

 ing with it a network of fragments. Such also is the case with the gran- 

 ite along the seaward shore of Marblehead, both north and south of the 

 Neck. The breaking of the syenite across the foliation, the interlock- 

 ing of its bands, its included fragments of schist, its irregular network 

 structure as well as its cross dikes, and the fact that, like the granite, it 

 sometimes covers large areas alone, — all this proves that it is an erup- 

 tive rock of later date than the schist, and not interstratified with it. 

 Furthermore, it was observed that the " Naugus Head Series " was cut 

 both by granite and felsitc, and that the statements to the contrary 

 were incorrect. In addition. to the syenite numerous dikes of diabase 

 occur of different periods, probably separated by long intervals of time. 

 (Proc. Bost. Soc. Nat. Hist., 1881, XXL, p. 294, Feb. 1, 1882 ; Har- 

 vard Univ. Bull, 1882, IL, p. 359.) 



Of the Huronian system Mr. Crosby remarks (Contributions, pp. 

 26, 27, 34, 36, 37, 38, 43,45): 



"The Huronian system in this region, like the Naugus Head series, though 

 in a soTucwhat less degree, exhibits great disturbance. Distinctly bedded 

 rocks are the exception ; and, although many apparently structureless rocks 

 are probably really stratiiied, it is undoubtedly true that a large part, perhaps 

 the greater part, of the formation has been more or less fluent, and extravasa- 

 tion may be set down as a characteristic structural feature The Huro- 

 nian series of Eastern Massachusetts is principally composed of the following 

 rocks, or, ratlier, groufis of rocks : — 



1. Granite (hornblendic and binary). 



2. Petrosilex (passing into iclsite and quartzite). 



3. Diorite (uustratified and largely exotic). 



4. Hornblendic Gneiss, Stratiiied Diorite, etc. 



5. Limestone. 



"Although so connected ]ithologically.and stratigraphically as to he clearly 

 members of one great series, yet these various groups are, on the whole, w-ell 

 separated, occurring mainly in large masses. The stratigraphic distinctness 

 would he much more striking hut for the wide-spread extravasation wdiich 



some of the divisions have experienced The true sequence, excluding 



some of the limestone, is expressed in the foregoing classification. 



*' The Huronian granites of this region sometimes exhibit traces of stratifica- 

 tion. Many examples of well-marked bedding have probably escaped obser- 

 vation ; and there can he little doubt that the granites in some cases really 

 possess a gneissic structure where the rock is too coarse and massive to enable 

 the eye to detect it. Yet I do not hesitate to assert that such phenomena 

 must, wherever occurring, he very local; for it can he proved beyond a doubt 



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