168 
GEOLOGY: R. W. SAYLES 
The manner of transition from the tilKte to the slate, together with 
other evidence, makes it certain that the slate was the result of deposi- 
tion in waters from the melting of the glacier which formed the tillite. 
The first of the transition beds are conglomerates alternating with sand- 
stones. Then come sandstones alternating with slates. At several 
places it is plain that the ice readvanced over these beds, ploughing them 
up and often dragging upward into the mass thus formed irregular 
lumps of the clay. As one goes upward in the banded slate it is seen 
that the layers of sandstone become thinner and thinner, then disap- 
pear entirely, and finally the alternating layers are shown only in slate 
of dark and light bands. 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 thicknesses also show regularity.^ The micro- 
scope also shows that the slate with the finest banding also has the 
finest particles of sediment, although the mineralogical composition is 
the same as in the rest of the slate and in the tillite. This finest slate 
however, has more chlorite than the rest of the slate, giving it a light 
green color. 
That the slate was derived from the tillite there can be no doubt. 
A microscopical examination by Prof. J. E. Wolff has shown the same 
mineralogical composition for both. The principal minerals present 
are as follows: quartz, feldspar, sericite, epidote, melaphyre, chlorite, 
limonite, quartzite and calcite. The size of the grains in the slate 
range from about 1 /300 of an inch to as fine as about one 1 /6000 of an 
inch. The shapes of the grains are angular as in ordinary glacial clay 
particles. As already stated the Hght bands usually have coarser as 
well as lighter material, but next to the thinnest sandstone layers coarser 
dark material is usually found, and this is just as it should be accord- 
ing to the laws of deposition. Thus it is seen that the banding in this 
slate is determined by the specific gravity of the material as well as by 
the strength of current. 
Identical mineralogical composition alone, however, would not prove 
that the slate was derived from the tillite, but two beds of tillite in the 
slate itself, together with the facts of the banding, prove this origin. 
The first bed of tillite is about 50 feet above the main tillite formation 
