1881-3 
215 
[Dodge. 
bencl somewhat within the small limits of their exposure to view. 
Not to reiterate familiar facts concerning the Brighton and Somer- 
ville dikes, this divergence may be illustrated by two sets exposed 
near the Neponset River which are less known. 
There are three dikes in the puddingstone at the Codman Street 
quarry in Dorchester. The direction of two of them can be fixed at 
about N. 60° W . One branches, including masses of puddingstone 
above and below which it unites ; the next (passing eastward) may 
be traced across the glaciated hill top, narrowing northward from 
nine inches at the southern end of its exposure to half an inch at 
the bottom of the quarry ; the third as at present exposed seems 
irregular, perhaps branching, and in many places has separated por- 
tions of quartz, calcite, epidote, chlorite, with numerous cavities. 
The slate west of Lower Mills is cut by two dikes one hundred 
feet apart, their direction varies but is not far from N.-S. They 
are both much decomposed. 
in part. Because others are not so, and because I had not in 1875 the means at hand of 
discriminating in the case of the finer grained rocks, even if it had been within my pur- 
pose to attempt this, I intentionally excluded the word pyroxene, and reserved ‘ hornblen- 
dic ’ (therein perverting the term, perhaps) as a generic word uncommitted to a speci- 
fic use, deeming it sufficiently accurate for the purposes of my paper, and hoped 
thereby at once to save periphrasis and to avoid tampering with the geological infer- 
ences involved in an assertion of the presence of one as contrasted with the other form 
of this mineral. 
The leading object of my paper was to encourage and aid search for fossils and 
study of the relations between the rocks, and I particularly desired to keep it free 
from terms which might fail to identify previously described objects to, and to be 
readily understood by, unscientific readers. This intention led me in several instances 
to choose an old familiar name in place of a better modern one. I did not anticipate 
misunderstandings on the part of competent critics. That my intention was not appar- 
ent resulted in some small part from the exclusion by the Publishing Committee of 
some prefatory explanations which originally constituted a part of the paper, and 
which in a modified form were restored in a pamphlet reprint. Nevertheless, the use of 
* hornblendic ’ (and some other words) seems to have required special explanation 
at the time, and I now desire to discredit some of its applications in that paper, which 
have not been commented upon, — in which cases, having been used as the substitute 
for certain symbols in a too inelastic system of field notation, the provisional use of 
these, while tracing the outcrops of the stratified group, years before writing, led to 
the employment of this unfortunate word where 1 basic,’ even ‘ eruptive,’ must have 
been as near to specific identification as was intended. I recognize, of course, the 
inaccuracy of the word ‘ granitic ’ in the sentence quoted by Mr. Wadsworth. 
