430 



THE AZOIC SYSTEM ^^T) ITS SUBDIVISIONS. 



a fluidal structure, and not due to a linear arrangement of the quartz 

 and feldspar, as had been supposed by some. The base of the felsito 

 has become devitrified, as is seen to have been the case, to a less extent 

 perhaps, in many modern rhyolites. The grains and crystals in the 

 groundmass extend from one band to another, and do not have a strati- 

 fied arrangement. Dr. Wadsworth's conclusions were published in 

 1879, in the Bulletin of this Museum (V., pp. 275-287). 



Dr. Wadsworth, not being able to find the time necessary for a thor- 

 ough examination of the felsites in the vicinity of Boston, with a view 

 to "a complete elucidation of the question whether these rocks were of 

 sedimentary or of eruptive origin, called for assistance on one of his 

 pupils, Mr. J. S. Diller, assigning to him, as a subject for a thesis,* the 

 felsites and the associated rocks in a district selected as being suit- 

 able for throwing light on the question at issue, and also because it was 

 one in which Dr. Wadsworth himself had done but little work, so that 

 the pupil would not feel in any way hampered by conclusions previously 

 reached by his teachers. All that was asked of Mr. Diller was to ob- 

 serve the facts carefully, and give the conclusions to which they led, 



let the results be what they would. 



Mr. Diller's work gradually expanded, until he took in finally almost 

 all of the felsitic area north of Boston. He made a detailed map of the 

 region examined, and published his results in the Proceedings of tho 

 Boston Society of Natural History ; and again, when further extended, 

 in the Bulletin of this Museum. Mr. Diller showed in the district 

 studied by him that a scries of stratified rocks occurred, which he re- 

 garded as the oldest rocks observed. These were intersected by dis- 

 tinctly eruptive granite, and this again by felsite. 

 through and through b^ successive eruptions of basaltic rocks. 



He concludes that there is no true felsite belonging to the stratilicd 

 group, the quartzite and slates being wholly distinct from the felsites 



It will be remembered that Mr. Crosby 

 stated that, on going northward in Mr. Diller's district, the porphyritio 

 character of the felsite gradually disappeared, the latter becoming inter- 

 stratified with quartzite and hornblende slates. It was said that the 

 transition was so gradual that no boundary line could be drawn between 

 the stratified and unstratified portions, thus proving that there was no 

 break and no natural division here. 



* Mr. Diller was at that time (1880-81) a candidate for the degree of S. B. in 

 Harvard University. Later he was aj^pointed Geologist to the Assos Expedition, and 

 is now in the employ of the XJ. S. Geological Sarvey. 



All were again cut 



both in structure and origin. 



