172 



BULLETIN OF THE 



by Dr. Wadsworth, as well as the numerous examples found upon the 

 coast of Marblehead Neck, conclude an array of facts which establish, 

 beyond dispute, not only that the felsite is a truly eruptive rock, but 

 also that in every case where we could deterniiue its relation it is 

 younger than the granite which it penetrates. 



The relation of the felsite to the stratified group is the subject of very 

 different opinions, and for this reason it seems proper to present in 

 detail the evidence we have observed bearing upon this question. If 

 the phenomena described sustain the conclusions already drawn, viz. 

 that the stratified rocks of Medford, Stoneham, Melrose, and Saugus 

 are older than the eruptive granites of that region, and that the felsite 

 is a truly eruptive rock younger than the granites, it is evident that 

 there can be no gradual passage from the felsite into the stratified 

 group. 



Mr. Crosby,* who has done so much on the geology of Eastern Massa- 

 chusetts, refers to the region west of the Boston and Maine liailroad in 

 Melrose as the one which "places beyond question the fact that there 

 is a gradual transition between the quartzite and petrosilex, and that 

 portions of the latter rock are intercalated in the stratified group." 

 This group is well exposed near the railroad, a short distance south of 

 the Melrose station, and extends so.uthwestward, parallel with the 

 general strike of the formation, across Spot Pond into Medford. Excel- 

 lent outcrops, perhaps the best of the stratified group in this region, 

 form a part of the northern shore of the pond. The felsites and strati- 

 fied rocks are exposed within several hundred yards of each other upon 

 opposite sides of Washington Street, Melrose, a short distance east of 

 the Melrose-Stoneham line. The exposures in both cases are within 

 the regions marked felsite by Mr. Crosby. Those upon the north side 

 of Washington Street are distinctly sedimentary rocks, while the rocks 

 in the cliOs to the southward arc well-marked felsite, slightly porphy- 

 ritic, of the kind forming the mass of the Maiden Highlands. Although 

 I have examined several times the region between Spot Pond and Long 

 Pond, where the stratified group meets the felsite to the southward, I 

 have not been able to find the two rocks in place exposed nearer to 

 each other than at the locality described ; and although in the present 

 state of our knowledge the existence of a *^ gradual transition " or inter- 

 calated felsite in the stratified group cannot be denied, it may be stated 

 that careful search has not revealed to us the slightest evidence in favor 

 of Mr. Crosby's positive assertion. It is hoped that the exposures 



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* Contiibutions to the Geology of Eastern Massachusetts, p. lOG. 



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