THE ROXBURY CONGLOMERATE SERIES. 47 
about fifty yards, to the next outcrop. Here the banding was seen to be very 
much finer. The outcropping of this very fine banding was continuous for 
about 100 yards and the banding at the highest observable part of the formation 
was extremely fine, the unit of fine and coarse layers not being over 1/32 of 
an inch thick. 
I have described what I saw during the examination of these two exposures 
at Squantum, and it took several visits to observe all the features spoken of in 
this description. 
STRUCTURE OF THE BANDING IN THE Suats. For the proper understanding 
of the origin and structural character of the banding in the slate the examina- 
tion of the deposits in the field must be supplemented by a study of microscopic 
sections. In a general way I have sketched the nature of the layers which make 
up the banding and other phenomena which are associated with the banding, 
such as contorted zones, tillite beds, etc. It remains now to describe the band- 
ing in more detail as it is seen in thin sections under the microscope. 
In a previous paper (1916, p. 168) I showed that the slate was surely 
derived from the tillite or from material of the same origin and character. The 
mineralogical composition of the tillite and slate is identical. My colleague, 
Prof. J. E. Wolff, examined specimens of both and found the following minerals 
in each:— quartz, feldspar, sericite, epidote, melaphyre, chlorite, limonite, 
pyrite, quartzite, and calcite. The sizes of the grains range from 1/12 mm. 
to as fine as 1/1100 mm. The shapes of the grains are angular as in ordinary 
fine, glacial sediments. The fact that the tillite and slate have the same minera- 
logical composition and shapes of particles, does not in itself prove that the 
slate came from the tillite. Two beds of tillite intercalated in the slate, how- 
ever, together with other evidence in the banding, prove this origin. 
In 1916, I described the general structure of the fine and coarse compo- 
nents of the banding of the slate as follows: 
“The microscope reveals the fact that the dark layers are composed of much finer 
material than the light layers. The coarse layers all, without exception so far as observed, 
have very fine wavy lines of bedding which are cut off and uneven in places, while the fine 
layers are solid in appearance without these characteristic lines. The finest part of the fine 
layer is usually in contact with the coarse layer upward, and the change from fine to coarse 
is abrupt. The change from coarse to fine is more gradual upward, as a rule, and not abrupt. 
These layers or bands alternate with much regularity and at any given horizon their thick- 
nesses also show regularity.” Sayius, 1916, p. 168. 
This description applies to the slate above the tillite where regular banding 
makes its appearance. (See Plate 16). Such characters also appear in some 
