52 SEASONAL DEPOSITION IN AQUEO-GLACIAL SEDIMENTS. 
between fine and coarse components. There is some question still as to the 
exact horizon of this slate. It may be the Dorchester slate member of the 
Roxbury series. The bands are thin, not over one eighth of an inch thick. 
They have been disturbed by wave or current action along the bottom as shown 
under the microscope by small ripple-mark, and by large size ripples found 
frequently at this locality. The water was evidently shallow, although deep, 
and quiet enough for the deposition of clays. 
6. BANDED SLATES WITH OTHER TILLITES. 
After working for nearly two years on the problem in hand, and convinced 
that I was the first to suggest seasonal banding in slate connected with tillite, 
it appeared that Dr. Thore G. Halle of the University of Upsala had antici- 
pated me by four years in the theory of seasonal deposition in the glacial slate. 
I had heard of Halle’s work on the tillite of the Falkland Islands, but was not 
aware that he had described the banded slate which occurs with the tillite, and 
American geologists generally seem to have overlooked the same. Halle after 
describing the tillite gives a detailed account of the banded slate. Part of his 
description is as follows: 
“ Another example of rapid changes in the sedimentation can be noted in a certain kind 
of claystone, which occurs in typical development near the settlement of Darwin Harbour. 
It is worked at this place in a small quarry near the beach, and is used for building purposes. 
The rock, which is nearly horizontally bedded, shows a marked lamination caused by the 
alternation of differently coloured zones. The alternation of the zones is a fairly regular 
one, one yellowish brown and one dark-grey zone, forming together one layer, or marking 
one cycle of sedimentation. The thickness of each stratum of two zones varies considerably, 
generally between some few millimetres and one centimetre. On a microscopical examination 
of a section of this rock it is seen that the brown or yellow zone is formed of much coarser 
material, angular or but little rounded quartz-grains being abundantly mixed in the denser 
matrix. The dark-grey zone, which often has a shade of green, consists of purer, fine argilla- 
ceous material. The whole appearance of this rock is strongly suggestive of the well-known 
seasonally laminated clays from the areas of Diluvial glaciation. In these clays, each stratum 
is generally understood to represent the sedimentation of one year. The coarser grained 
brown or yellow zone, formed during the melting of the ice in spring and summer, passes 
gradually into the dark-grey zone corresponding to a slower sedimentation, and borders this, 
with a sharply-defined limit, on the lower zone of the following year. In the claystone of 
Darwin Harbour the limits between the zones are not so regularly marked, and it is not 
always possible to distinguish between a gradual change from brown to grey, and the rapid 
one from grey to brown. Yet, the resemblance to the real seasonal clays is great, and it 
seems probable that this Permo-Carboniferous clay owes its peculiar lamination to a succession 
of annual layers. At any rate, a certain periodicity in the sedimentation is undubitable.” 
Hate, 1912, p. 159, 160. 
