MUSEUM OF COMPARATIVE ZOOLOGY. 



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which is the product of alteration. The composition of this interesting 

 rock, and its potrologic relations, wc believe, establish beyond question 

 that it is a veritable ancient volcanic ash. There is considerable 

 evidence, though not yet decisive, for want of further microscopical inves- 

 tigations, that these ashes are distributed in patches througliout a con- 

 siderable portion of the region occupied by the felsites. 



It seems to be certain, therefore, tliat there has been within this re- 



r 



gion a true volcanic outburst by which the ashes were produced, and 

 that anterior to the formation of the ashes, or perhaps about the same 

 time, there was an eruption of felsitic hiva, with which the ashes became 

 entangled in the complicated manner we find upon Break-heart Hill. 



The banding so well marked in the felsites upon the western shore of 

 Marblehead Neck until (piite recently has been considered stratification, 

 and therefore appeared to be a fatal argument against the theory of 

 those who regard the felsites as exotic. Dr. Wadsworth was the first to 

 consider the banding a fluidal structure, equivalent to that so prominent 

 in the modern rhyolitcs, and consequently of igneous origin. It is of 

 common occurrence, especially in the silicious felsites of Lynn, Saugus, 

 and Melrose ; and from the fact that where it is found in felsite, forming 

 distinct dikes or tongues in the granite, or enveloping fragments of other 

 rocks, the banding is parallel to the line of contact, there seems to be no 

 good reason for doubting that it was produced by the flowing of the fel- 

 sitic matter in a state of fusion. The extreme complexity of the band- 

 ing upon Break-licart Hill and at places in Marblehead Neck cannot be 

 satisflictorily explained by any other supposition, and it is scarcely nec- 

 essary to add that under the microscope the phenomena of the banding 

 are wholly at variance with those found in sedimentary rocks and com- 

 pletely in harmony with those of altered rhyolites. The banding, there- 

 fore, instead of furnishing an argument in fivor of the sedimentary 

 origin of the felsites, is proof of tlieir eruptive character. 



Since the relation of the felsite and granite has been the subject of 

 such discrepant statements, we shall point out the localities in which, as 

 it seems to us, the relation is clearly exposed. Within the granite 



southwest of Spot Pond a dike of somewhat piidcish felsite occurs near 

 the top of a high hill west of the north end of Brooks's Lane, where it 

 enters Forest Street. Further northward and nearer to Forest Street 

 there are, at least half a dozen smaller dikes of a similar felsite in the 

 granite. That these masses of felsite are true dikes and not included 

 fragments witliin the granite is proved by the fact that they not only 

 scud irregular tongues of felsite from tha main dike into the adjoining 



