wai.cott.] SUMMARY MASSACHUSETTS. 269 



The deposition of the Primordial strata was followed by a period of disturbance of 

 this part of the earth's crust, during which they were strongly compressed, being 

 thrown into sharp folds having, in general, a northeast and southwest direction. This 

 appears, also, to have been a period of intense volcanic activity. The quartzite and 

 slate were shattered and isolated by great volumes of basic lava, which we now call 

 diorite; and, as indicated by the compact and scoriaceous diorite of the Middlesex 

 Fells and other localities, immense floods of lava were also poured out on the surface, 

 concealing the sedimentary rocks. In breaking through the quartzite and slate, the 

 diorite has often followed the planes of stratification in those rocks, and thus appears 

 in many cases to be regularly interstratified with them ; and this bedded appearance 

 of the diorite is greatly increased by the very perfect flow-structure which was de- 

 veloped in large masses of it, as well as by the distinctly schistose or foliated struc- 

 ture sometimes resulting from the subsequent action of both mechanical and chemical 

 forces. We are thus able to explain the fact that the diorite has been frequently 

 mistaken for a stratified or sedimentary rock. 



The eruption of the diorite was probably followed by a prolonged period of quiet 

 erosion, which was finally terminated by the advent of a second period of intense 

 and long continued igneous action, during which only acid rocks — the granites and 

 felsites— were formed. All over this region the granite has broken through the Pri- 

 mordial strata and the diorite in the most irregular and intimate manner. 



Prof. Crosby considers that after the deposition of the argillites and 

 the eruption of the rocks occurring as dikes the series was raised above 

 the sea level and subjected to long-continued erosion, prior to a down- 

 ward movement or subsidence. As the surface passed slowly below 

 the level of the sea a thick bed of conglomerate was spread over the 

 earlier series of rocks and. a fine silt or clay carried out into the deep 

 water of the ocean. At the same time eruptive rocks were repeatedly 

 poured out over the sea floor, where the beds of gravel and sand were 

 accumulating, thus forming alternatiug beds of conglomerate and sand- 

 stone with beds of mela^hyr and porphyrite. 1 



Of the subsequent depositions of sediment, he says : 



Alternations of the conglomerate with beds of both sandstone and slate are also of 

 common occurrence, indicating oscillations of level during this period. But the down- 

 ward movement prevailed, until finally the water became too deep and quiet and 

 too remote from the shore, in this vicinity, to permit the formation of conglomerate 

 and sandstone; but these coarse sediments were gradually replaced by slate during 

 the dying out of the volcanic activity. These tranquil, deep-sea conditions must 

 have continued for a very long time, for the argillaceous sediments accumulate very 

 slowly and yet the slate series has a thickness of from 500 feet to fully 1,000 feet or 

 more. The slates are probably somewhat thicker than the conglomerates, and were 

 doubtless several if not many times longer in forming. Of the life existing in the sea 

 at thistime we unfortunately know but little, as the slate is very generally destitute 

 of fossils; and the full chronologic significance of the few organic remains that have 

 been obtained from the layers of limestone in the slate at Nahant is still undeter- 

 mined. 2 



There is evidently an error in Prof. Crosby's interpretation of the 

 strati graphic succession of the formations he describes, as the fauna at 

 Nahant is of an older date than that found iu the argillites at Braintree. 

 If, however, the conglomerates, with their contemporaneous eruptive 



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1 Op. cit., p. 20. »Op. cit., p. 21. 



