EASTERN MASSACHUSETTS. 429 



they ai'e very fairly represented by the modern abyssal accumulations, espe- 

 cially if we take into account the enormous period of time Avhich has elapsed 

 since their formation, and the probable changes in the physics and chemistry 

 of the sea which it has wrought. The petrosilicious rocks are often distinctly 

 and beautifully banded. This structure usually results from the alternation of 

 very thin and regular quartzose and feldspathic layers, and although doubtless 

 originating in, and determined in direction by, the sedimentary process, I think 

 it can be proved that it has been made much sharper and more definite by a 

 subsequent partial segregation of the ingredients, especially the Silica." 



Dr. Wadsworth, from examinations of the felsites in the field and 

 under the microscope, became convinced, in 1878, that their characters 

 were the same, excepting in their alteration, as those of the volcanic 

 rhyolites so common in the Cordilleras, the banding being a fiuidal 

 structure. He convinced himself that we had here the remains of 

 ancient volcanic action, shown by ashes and other ejected fragmental 

 materia], lava flows, dikes, etc., such as at the present day accompany 

 eruptive action. Subsequently the whole series had been more or less 

 altered by various ageucies. While in some places a recomposed con- 

 glomerate was found made up of water-worn detritus resting on the 

 parent felsite, the structure as a whole was unlike that described by 

 Messrs. Hyatt and Hunt. 



Dr. Wadsworth found that in many cases there had first been thrown 

 out a rhyolitic ash, and that through, around, and over this ash the 

 rhyolitic lava has been poured, — the structure being identical with 

 that observable in the more recent lava flows of the West. No passage 

 could be traced between the fragmental and non-fragmental forms, but 

 distinct lines separating the two could be found on careful observation. 



Dr. Wadsworth also ascertained that two or more distinct flows had 

 taken place at Marblehead Neck, and that the felsite also cut the granite 

 in dikes of various dimensions. The opposite view had been taught by 

 Messrs. Hunt and Crosb}', but their evidence was taken from some fel- 

 site dikes cutting the granite near the boundaries of the main masses of 

 these rocks, where only the most careful examination would show which 

 was the intrusive one. Dr. Wadsworth's statement was based on find- 

 ing smaller dikes of felsite, which gradually narrow and come to an end 

 in the granite and on felsite dikes which hold fragments torn from their 

 gi'anite walls. He has examined the relations of the two rocks over 

 much of the coast, and has always found the same relation between 

 them. 



As a result of microscopic investigations the banding was found to be 



