11 
[Annual Meeting. 
relating to the composition and structure of the hard rocks, as well 
as to the drift deposits and other superficial features, we are now 
prepared to correlate the facts which have been observed and de- 
duce from them, so far as may now be possible, a connected and 
logical statement of the leading events in the history of this region. 
The oldest rocks which we have found are the Primordial slates 
and quartzites ; and the age of these is certainly and definitely 
known only at the Paradoxides quarry, in Braintree. We appear 
to be justified, however, in regarding them, provisionally at least, 
as all of about the same age, partly on account of a general litho- 
logic resemblance, but mainly because their relations to the different 
classes of eruptive rocks are everywhere the same. In Weymouth 
and Braintree, where we first met these rocks, they are either typ- 
ical clay slates or slightly calcareous ; but along the northern base 
of the Blue Hills occasional layers are distinctly siliceous. They 
probably underlie a large part of the Boston Basin, being covered 
by the conglomerate and the newer slate ; and north of the basin, 
as shown in the ninth lecture, they occur in isolated areas among 
the eruptive rocks. In some of these areas, especially in the Mid- 
dlesex Fells and Melrose, and in Woburn, clay slate similar to that 
in Quincy and Braintree is repeatedly inter stratified with quartzite ; 
while toward the southwest, in Natick, and also in Reading and 
Lynnfield, there are extensive developments of quartzite with little 
or no slate. It is very clear that the quartzite north and west of 
the Boston Basin is the source of the quartzite pebbles which play 
such a prominent part in the composition of the conglomerate, es- 
pecially in the central and northwestern sections of the basin. In 
general, the quartzite is more and the slate less abundant north- 
westward, indicating that the ancient shore-line along which these 
strata were deposited lay in that direction ; and originally the 
Primordial strata were probably spread continuously over all the 
region to the southeast of that line. 
The deposition of the Primordial strata was followed b}^ a period 
of disturbance of this part of the earth’s crust during which they 
were strongly compressed, being thrown into sharp folds having, in 
general, a northeast and southwest direction. This appears, also, 
to have been a period of intense volcanic activity. The quartzite 
and slate were shattered and isolated by great volumes of basic 
lava, which we now call diorite ; and, as indicated by the compact 
and scoriaceous diorite of the Middlesex Fells and other localities, 
