168 BULLETIN OF THE 



Pelsite. 



The felsites, extending from Medford through Maiden, Melrose, and 

 Saugus to the eastern part of Lynn, and northward from Melrose into 

 Wakefield, are the prevailing roclis of that region. Their petrological 

 relations have been one of the chief enigmas in the geology of Eastern 

 Massachusetts, and, judging from the present diverse opinions, much 

 thorough work needs to be done before a final solution is reached. Most 

 observers agree that the felsites are of more recent age than the granites ; 

 nevertheless, there is a wide difference of opinion concerning the phe- 

 nomena upon which this common conclusion is based. Some observers 

 maintain that the felsite is younger than the granite, from the fact, as 

 they say, that the felsite not only envelops detached fragments of the 

 granite, but also penetrates it, in the form of distinct dikes. Other ob- 

 servers reach the same conclusion, because, in their opinion, the granite 

 envelops felsitic fragments, and in the form of irregular dikes pene- 

 trates the felsite. 



It is evident that theoretical considerations are of little value unless 

 supported by facts, and for this reason the latter should be clearly set 

 forth and particularized, apart from theories, so that there may be some 

 hope of securing, ultimately, a uniformit}' of opinion concerning at least 

 what occurs in the field. 



As far as our observations have extended, we have never seen an exam- 

 ple of the granite breaking thi'ough the felsite or enveloping its frag- 

 ments. In every case in which we could determine the relations of the 

 rocks the felsite occurred as an eruptive penetrating the granite. 



Before proceeding to point out the special localities in which the gran- 

 ite is cut by the felsite, let us consider the evidence, apart from the dikes, 

 which proves the truly igneous nature of the latter rock. 



Upon Break-heart Hill in Saugus and to the west towards Main Street 

 there occurs an extremely heterogeneous mixture of banded felsite with 

 grayish fragmental material, which in its external aspect resembles, to a 

 considerable extent, ordinary ashes. The true nature of this grayish 

 rock was not suspected until the microscope revealed the fact that it 

 contains many of the splinter-shaped or chip-like sharp-edged fragments 

 whose peculiar forms belong to the volcanic glass of rhyolitic tufas. The 

 forms are so characteristic, and in this case so well preserved among the 

 other ashy material, that there can be no doubt of their identity. The 

 fragments, as we would expect,* are no longer volcanic glass, but quartz, 



* On the Classification of Rocks, hy M. E. Wadswoith ; Bull. Mus. Conip. Zool. 

 at Cambridge, Mass., Vol. V., No. 13. 



