1881 .] 
293 
[Wadsworth. 
since the deposition of the Braintree Primordial argillite, but 
how soon thereafter or how long continued, there is as yet no 
evidence. Studies of these rocks in more recent years have devel- 
oped the facts, that they occur not only in dikes but also in lava 
flows ; and that in many cases they are formed out of consolidated 
volcanic ashes. Mr. J. S. Diller in 1880 made the remarkable 
discovery of pseudomorphs after shreds of volcanic glass. These 
forms are so peculiar and remarkable, that they add evidence of 
the most ]oositive kind, to that already known, proving in this dis- 
trict the presence in former times of volcanic action, like that 
which has taken place in modern times upon the earth’s surface. 1 
The structure of the region is similar to that which would be 
observed in any volcanic district, after it had been subjected to 
the various agencies this has been exposed to during countless 
ages. Most of the workers in Eastern Massachusetts have studied 
with preconceived theories as to the origin and nature of the 
rocks ; this added to the obliteration, through various causes, of 
the superficial features of eruptive (including volcanic) rocks, 
has in general prevented sufficient study from being given to 
ascertain the true character of the formations. But comparatively 
few observers have worked in Eastern Massachusetts from the 
standpoint of simply interrogating the rocks themselves, and find- 
ing out what answer they could return to the questions of their 
origin and history. 
The prevalent custom of working in regions of crystalline 
rocks, with preconceived theories as to their origin and history, has 
prevented elsewhere, in the opinion of the writer, the, actual obser- 
vance of the true characters displayed by the rocks in question. 
This added to obliviousness to the facts, that the same rocks 
are produced by eruptive agencies in all ages, and that of all rocks, 
eruptive ones (including dikes, lava flows, ejectamenta, and 
derived detritus) are the most subject to metamorphic action, has, 
it seems to the writer, been the cause of the numerous subdivi- 
sions of the Azoic rocks, and the assignment of rocks to those 
subdivisions on lithological evidence alone. 
1 Proc. Bost. Soc. Nat. Hist. 1880, xx, 355-368, 416-426 ; 1877, xix, 217-237, 309- 
316. 
Bull. Mus. Comp. Zool. 1879, v, 283, 1881, vn, 165-180. 
