48 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
of the slate under the tillite. It might be added that the same characters appear 
in the slate at most other places where the Cambridge or Dorchester formations 
oecur. The localities in question from which slate has been examined micro- 
scopically are:— Crow Point, in Hingham, Newton, Brighton, Cambridge, 
and Somerville. These slates will be discussed later. After two years’ observa- 
tion, it is not necessary to change the above description of the typical characters 
of the banding. Various irregularities intrude themselves, and await explana- 
tion and these will be considered in their proper places. 
As has been stated, the fine components of the banding begin gradually to, 
grow finer and continue to become finer in texture, until at the top the very 
finest material of all is found in a well-defined layer. (See Plate 16, fig. 2). 
This topmost layer frequently has microscopic ripple-mark on its upper surface, 
indicating the change to conditions of water in motion. Sometimes this almost 
universal layer of extremely fine sediment by which it is always possible to 
recognize top from bottom, is absent. In such rare cases it is evident that 
water currents of the high-water phase have washed this finest sediment away. 
Such currents might not have a velocity of over four inches a second but that 
would be enough to erode the finest clay. The sizes of the particles of the fine 
components average about 1/800 mm. in diameter with here and there larger 
fragments. The finest fragments are about 1/1100 mm. in diameter. 
In the midst of the fine components there sometimes occur very thin layers 
of coarser material. This might mean a thaw in a winter season with resulting 
higher water. In the middle of the slate formation the fine components some- 
times show these intercalated coarse layers. 
The coarse components of the banding do not exhibit such a compact and 
homogeneous nature as the fine. As has already been stated, they consist of 
alternating layers or remnants of layers of coarser and finer sediment and 
although the fragments are fine, with some as fine as any found in the fine com- 
ponents, the material as a whole is much coarser than that in the fine components. 
(See Plate 16). Grains as large as 1/12 mm. are common, although grains of 
this size do not make up the bulk of these components. From the broken 
nature of the layers it is clear that deposition and erosion went on alternately. 
What was deposited on one day might be mostly eroded the following day. A 
count of these alternations, using all the sections I have at present, gives an 
average of about forty for a coarse component. They run as high as sixty and 
as low as twenty-four. The continual erosion of layers deposited would destroy - 
the complete record of the alternations. The resulting coarse components of 
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