1890 .] 
13 
[Annual Meeting. 
The different varieties of felsite, like the granites, afford evidence 
of having been erupted at several distinct but probably not widely 
separated periods. The granite and felsite are, next to the quart- 
zite, the principal constituents of the conglomerate, their greater 
durability placing them far ahead of the diorite in this respect. 
Probably every known variety of both the granite and felsite is repre- 
sented in the pebbles of the conglomerate ; and it is certain, there- 
fore, that the eruption of these acid rocks was followed by a long 
period during which this region was dry land and suffering erosion, 
before the deposition of the conglomerate began. General consid- 
erations with regard to the climate of that early epoch in the history 
of the Boston Basin, as well as the absence of pebbles of diorite 
from the conglomerate, indicate, as Mr. Bouve has so clearly point- 
ed out, that the wearing away of the hard rocks was due very largely 
then, as it is now, to chemical action. The diorite and other basic 
rocks, as is now so well illustrated by some of the dikes about Bos- 
ton, yield far more readily than the highly acid rocks — quartzite, 
granite and felsite — to the chemical agency of air and water, and 
are, therefore, more generally and completely reduced to the con- 
dition of an impalpable clay or soil. 
An important and long continued downward movement or sub- 
sidence of this part of the earth’s crust finally began ; and as the 
surface slowly passed below the level of the sea a thick bed of 
conglomerate was spread over this region, the products of the pre- 
vious chemical decay of the rocks being rapidly worked over by 
the waves and the fine silt or clay carried out into the deep water 
of the ocean, while the coarser materials were strewn along the ad- 
vancing beach. As has so frequently happened in geological history, 
this movement of subsidence and the rapid deposition of sediments 
was accompanied by volcanic activity. The eruptions of this period 
consist chiefly of surface flows ; and they are of a distinctly basic 
character, melaphyr being the prevailing type, although porphyrite 
and possibly still more acid lavas are also observed. These were 
probably mainly crater eruptions, occurring in and near the sea ; 
but we have not as yet been able to definitely locate any of the 
vents. It is certain, however, that floods of liquid lava were re- 
peatedly poured out over the sea- floor where beds of gravel and 
sand were forming ; and we thus find, as at Nantasket, Brighton, 
etc., beds of conglomerate and sandstone alternating with beds or 
flows of melaphyr and porphyrite ; but in some cases, as in Hing- 
