1881 .] 
205 
[Dodge. 
which lies between Warren and Washington Streets, and east- 
ward, low as the ground is the rock surface is lower yet and there 
are few or no outcrops. On the south side of Warren Street, 
west of Breck, is conglomerate which gives place to sandstone, 
and this latter affords a Avell known outcrop with ripple marks on 
its broad slabs half way between Breck and Cambridge Streets. 
This rock also forms one or two ledges on the north side of War- 
ren Street, opposite the conglomerate exposure last named. 
Brighton Amygdaloid '. The quarter of a mile next succeed- 
ing to the northward, includes the well known amygdaloid local- 
ity. Sandstone, slate and puddingstone extend across this district, 
but are much displaced by the intrusive rock. The strata and 
the intrusive masses, which at times have an irregular bedding, 
may be seen in all positions with reference to one another, the 
stratified rocks being underlaid, overlaid, included and vertically 
cut across by the other. Where the slate overlies the amygdaloid 
the former is penetrated by numerous veins of quartz which seem 
to find their source in the latter. Dikes of a later jjeriod, as is well 
known, cut this amygdaloid as well as the slates and sandstones 
with which it is connected. 
The causes which have determined the present position and 
shape of the exposed portions of these rocks are few and simple. 
The south-eastern and south-western sides of ledges are usually 
steep, conforming to the direction of joints. The two principal 
horizontal directions of these joints are about 1ST. 30° E., and N. 
25° W. The glaciated sides of ledges, on the other hand, 
slope gradually upward. Several narrow ridges occur having a 
direction of about N. 55° W. — S. 55° E. ; the north-easterly 
sides of these were of course subject to glacial action (operating 
in a direction S. 35° E.), and consequently are smoothly sloped, 
while the south-western sides retain the steeper joint surfaces. 
The dips of the slates and sandstones are to the northward and 
north-westward from 20° to 35°. The strike, which may be re- 
garded as normally between E. — W. and E. N. E. — W. S. W., 
varies over a range of 100°. 
At several places, glaciated surfaces of the amygdaloid show an 
interesting feature of its structure. Irregularly rounded or pear- 
shaped patches are noticeable upon such surfaces, whiter than the 
