178 
SUPPLEMENT. 
During the time that has elapsed since the printing of this 
work began, some additional observations have been made 
which, although not involving any material changes in my 
views concerning the geological structure of the region, it is 
thought best to indicate briefly, in order to make the exposition 
of facts as complete as possible. 
Besides the two small granite quarries on the Cohasset and 
Scituate shores, referred to on page 17, there is a third quarry 
on the south side of Scituate Hill which has afforded some 
good stone for general use. 
Undoubtedly the most interesting flow of melaphyr in the 
Nantasket district is the second melaphyr in the Valley Beach 
section described on page 52. This flow is of particular 
interest, not alone because it can be traced in one direction to a 
distinct edge, but also on account of the numerous, irregularly 
rounded, amygdaloidal masses which it incloses. I have referred 
to these masses as pseudo-bombs ; but, although well satisfied, as 
stated, that they are not true volcanic bombs, or projected 
masses of lava, it is very gratifying to find this view confirmed 
by the observations of Prof, J. D. Dana on the lavas of the 
Hawaiian Islands; and I am grateful to Mr. Geo. P. Merrill 
for calling my attention to the following statements by Pro- 
fessor Dana.! 
The aa streams are remarkable also for the presence of lava- 
balls of concentric structure that have been wrongly called 
bombs. These lava-balls are smoothish exteriorly, more or less 
rounded and bowlder-like, and vary in size from an inch or less 
to ten feet and more. 
Some of these lava-balls have, outside, a crust of hard lava, 
and, inside, fragments of scoria; others consist of concentric 
shells, hard and scoriaceous shells alternating with one another. 
! Amer, journ. sci., 1887, ser. 3, vol. 84, p. 364. 
