1880.] 8367 [Diller. 
all of the same age. The eruption of some of the felsites occurred 
before the formation of the breccia, and after the breccia was formed 
another eruption occurred. 
The passage of the breccia into sandstone and conglomerate indi- 
cates the sedimentary origin of the breccia. 
The eruption of the ‘“ basic felsite’’ probably occurred after the 
formation of the breccia. 
Supplementary Note. 
Since the above paper was read I have examined a number of 
new thin sections of brecciated rocks in Malden and Saugus, and 
discovered some facts which have a very important bearing upon the 
origin of the felsites. 
Prof. Zirkel in his report upon the rocks of the Fortieth Parallel, 
has described, under Younger Clastic Rocks,! some which contain to 
a greater or less extent splinters and chip-like sharp-edged fragments 
of volcanic glass. 
Some of the rhyolitic tufas of California contain a great number 
of these peculiar conchoidal fragments, which, according to Mr. M. 
E. Wadsworth,? are identical with the splinters, chips and shards of 
glass described by Zirkel. The forms of the glass fragments differ 
very markedly from those of the other fragmental material, and at 
once become a very characteristic feature of clastic rocks containing 
fragments of volcanic glass. 
About a mile north-east of Oakland Vale, on the north side of 
the stratified group, is an area of brecciated rocks. One of the thin 
sections from these rocks shows a large number of the peculiar 
voleanic-glass forms, and is incontrovertible evidence of the former 
presence of volcanic glass, which must have been brought to the 
surface by voleanic action. 
Mr. Wadsworth has examined the thin section and accepts the 
facts as conclusive evidence of the former presence of fragments of 
1Geological Explorations of the Fortieth Parallel. Vol. Iv., Microscopical 
Petrography, pages 265-271. 
2 Mr. Wadsworth examined the thin sections described by Zirkel in his report 
upon the rocks of the Fortieth Parallel. See Mr. Wadsworth’s paper ‘‘On the 
Classification of Rocks.’ Bulletin of the Museum of Comparative Zoology, 
Cambridge, Mass, Vol. v., No. 13. 
