Relations of the Igneous Rocks — Crosby. 69 
GENETIC AND STRUCTURAL RELATIONS OF THE IGNEOUS 
ROCKS OF THE LOWER NEPONSET VALLEY, 
MASSACHUSETTS. [II.] 
By- W. O. Crosby, Boston, Mass. 
PRE-CARBONIFEROUS INTRUSIVES. 
The earlier or pre-Carboniferous intrusive rocks of the 
batholite include the following types : granite porphyry, 
quartz porpryry, felsite and acid andesite. The first three 
are acid and agree closely in composition, as stated above, 
with the sedentary types of the batholite, so that they may 
fairly be regarded as later extravasations from the deeper 
parts of the same great body of magma. The fourth type, 
on the other hand, is of distinctly sub-acid or neutral com- 
position, as a partial analysis accompanying Dr. Bascom's 
description clearly shows. Furthermore, the dikes of this 
relatively basic type are, according to the rather meagre 
but quite satisfactory evidence, older than all of the acid 
intrusives, suggesting eruption from a source below the 
normal granite and hence during a time (possibly of mark- 
ed elevation and erosion) when the batholite proper was 
congealed throughout its entire thickness. 
The acid intrusions, on the contrary, including both 
dikes and necks, may, in the main at least, as previously 
indicated, be correlated with the subsidence ushering in the 
sedimentation of Carboniferous times and accompanied by 
a rise of the isogeotherms sufficiently marked to reliquefy 
a portion of the ancient acid magma. This correlation is 
confirmed by the close agreement in composition of the 
acid intrusions and the normal granite. — for the unquestion- 
able connection of the acid intrusives (both dikes and 
necks) with the acid effusives or lavas, which rest uncon- 
formably upon the deeply denuded surface of the batholite, 
shows that in origin the intrusions are separated from the 
sedentary zones of the batholite by a vast time interval 
during which the magma must have been mainly solid to 
have escaped marked chemical differentiation. 
Acid Andesite Dikes — This is a rock of distinctly neu- 
tral composition, anlyses showing from 60 to 62.8 per cent. 
of silica ; and Dr. Bascom's observations indicate a horn- 
blende-bearing biotite andesite. A single dike only has 
