1 10 The American Geologist. Februar.v 899 
Cambrian localities of Nahant, Weymouth, and Braintree, he 
first two Lower Cambrian and the last Middle Cambrian. 
These are probably all of Cambrian age. The Basin series is 
terminated abruptly on the west by a fault. 
In the small area west of Waltham, we have a definite 
stratigraphical series beginning at the bottom with a quartzyte 
member which for convenience I have called the Stonybrook 
quartzyte from the locality where it is best exposed, a method 
of naming generally followed. This member has a dip of 
about 60-70 degrees N. W. and a strike about N. 40-50 E.. 
with thickness of at least 500 feet. The base rests on a red 
porphyritic granite; a red feldspathic element in the quartzyte 
indicating an arkose. The granite which contains large 
phenocrysts of orthoclase, includes masses of diorite and upon 
closer study Vv'ill undoubtedly be found to consist of separable 
members here considered as a unit and placed* provisionally 
with the Archaean. The quartzyte becomes exceedingly fine 
grained in the upper portion and it may yet be found neces- 
sary to separate this portion as a silicious or magnesian slate. 
Xumerous small faults make the stratigraphical relations ex- 
ceedingly complex. The upper portion of the quartzyte shows 
an excellent alternating passage into the next member, the 
Kendal Green slate. This agrees in dip and strike with the 
first, but attains a much greater thickness, estimated at prob- 
ably over 5,000 feet. It is in the main a schistose slate, using 
the term slate in its widest sense, consisting of hornblende, 
quartz and many other constituents, the entire description of 
which would be too great a task for the scope of this paper.* 
The primary constituents vary in the direction of dip, owing 
to variations in the conditions of sedimentation. The original 
deposit must have been thickly bedded since the stratification 
is easily lost in process of metamorphism. The upper por- 
tion marks a return of the conditions of the coarser materials 
of tlie underlying formation. 
The third member of the series is the Lincoln slate. This 
has the usual dip and strike, one of the marked feature of the 
*The reader is referred with pleasure to the papers published by Mr. 
John Sears in the Bulletins of the Essex Institute of Salem. Mass., 
for petrographical descriptions of similar rocks from an area to 
the northeast of that of the writer which is on the line of strike of the 
formations described here. 
