EASTEIIN MASSACHUSETTS. 



425 



Mr. Crosby also states emphatically that " this rock [the granite of 

 Eockport] is destitute of mica, or at least its presence is a very rare 

 occurrence." Dr. Wadsworth showed in 1878 (Proc. Bost. Soc. Nat. 

 Hist., XIX., pp. 309~31G) that this was an error, so far as it concerned 

 at least ninety-five per cent of the rock then quarried at Kockport. It 

 was also pointed out, that a brief inspection of the two buildings in 

 which Mr, Crosby had for many years been working would afford evi- 

 dence of the mistake, as would also an examination of the buildings on 

 almost any street in Boston. The correctness of these statements of 

 Dr. Wadsworth was acknowledged by Mr., Crosby in 1880 (Crntribu- 

 tions, p. 28). 



Of the felsite Mr. Crosby writes (Eeport on the Geological Map of 

 Massachusetts, 1870, pp. 17-21): 



" In general it is a structureless rock, showing no trace of bedding ; but at 

 Dungeon Rock in Lynn, it is distinctly stratified, a dense, black variety being 

 interstratified with a crystalline dioritic variety ; and going northward in 

 Melrose, the porphyritic character gradually disappears, the felsite becomes 

 more siliceous, and gradually becomes interstratified with quartzite and horn- 

 blende slates. The, transition is so gradual that it is impossible to define 

 the boundary between the stratified and unstratified felsites, which proves 

 there is no break, no natural division here. In Melrose and Maiden, and at 

 other points the porphyritic felsites exhibit frequent local passages into granite 

 and (liorite. It seems probable that considerable portions of this rock have 

 been in a more or less fluent state, this can scarcely be doubted on the west 

 shore of Wenuchus Lake in Lynn, where the tongue of granite peuetrates the 

 felsite ; and on Marblehead Keck and the neighboring islands, there is abun- 

 dant evidence of-the softening and extravasation of portions of the rock. Nota- 

 bly on Marblehead Neck, also on Red Rock in Lynn, and at the Pirate's Glen 

 in Saugus, and, perhaps, at other points, this felsite exhibits traces of a con- 

 glomerate origin There are in this region two principal varieties of 



* banded ' or laminated felsite, which differ widely in their origins ; first, that 

 in which the banding is due to a conglomerate origin, having been produced 

 by a fiattening of the pebbles of the conglomerate ; . . . . and second, the 

 much more abundant and widely distributed variety in which the banding 



represents the original bedding of the rock The banding commonly 



results from the iuterlamination of thin layers of quartzose and feldspathic ma- 

 terials. The thickness of the lamina) usually varies from a mere line to one- 

 sixteenth of an incli, and seldom exceeds one-eighth of an inch. That this 

 banding really represents stratificoation is proved by the regularity and continu- 

 ity of the bands, since a banded structure due to the flattening of the pebbles 

 of a conglomerate would necessarily exhibit little uniformity in the thickness 

 of the lamina), and I find it dilficult to conceive of pebbles flattened to such an 

 extent as to produce continuous layers of uniform thickness and yards in 



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