THE ROXBURY CONGLOMERATE SERIES. 43 
in the first description, there is left about 180 feet of tillite proper. There 
was also one thin bed of alternating slate and conglomerate layers, three feet 
thick, which escaped me in the first description. 
The sequence in the tillite formation, as near as I have been able to make it 
out, is as follows:— The Dorchester slate underlies the tillite at Squantum. 
The strike of the slate is N. 48° E, and the dip 25° S. The first tillite bed is 
not over twelve feet thick. Above this comes three feet of alternating slate 
and conglomerate layers. On these layers there is about fifty feet of tillite 
followed by a bed of sandstone about twenty feet thick. Above the sand- 
stone is a bed of tillite about eighty feet thick. Following this tillite a bed of 
conglomerate about fifteen feet thick, as seen in the old quarry on the hill. On 
this conglomerate is about fifty feet of tillite, with about twenty feet of alter- 
nating coarse and fine transition beds above it, and merging into these above 
comes the main slate formation, to be. described. The thicknesses and the 
sequence given above are still provisional and must not be taken as final. Nothing 
but a trench through the hill will ever settle beyond question the sequence of 
the beds of this. formation. 
The tillite farther south at Squantum, described in the ‘‘Squantum tillite” 
under the heading ‘‘Squantum Southeast” was thought at that writing to be 
the same tillite formation as that found at Squantum Head. It may be, and 
only by a study of the structure of the whole basin can this point be decided. 
It should be noted that the tillite series at Squantum southeast is between 500- 
600 feet thick and little over 250 feet thick at Squantum Head. Such a great 
difference in thickness in such a short distance may be possible, but further 
study of the structure will be necessary to settle this point. 
Tur Campripgs Suate Mermpsr.' Emerson describes the Cambridge 
slate as follows: 
“The Cambridge slate, named from Cambridge, where it has been encountered in many 
excavations, consists of perhaps 3,500 feet of slate, shale, argillite, and some interbedded 
sandstone, and at or near the top about 40 feet of greenish and yellowish quartzite. Beds 
here and there are composed of reworked tuff. The formation is of rather uniform lithologic 
character, and appears to have been deposited in a body of fresh water, possibly a lake at 
the margin of the ice.” Emerson, 1917, p. 57, 58. 
The uppermost exposure of the slate at Squantum, ‘probably not more 
than 800 feet above the tillite, has a greenish tint due to a large amount of 
chlorite. This large percentage of chlorite in the slate might indicate that 
1 The slate south of Squantum Head is probably the Cambridge slate. 
Greet se 
