‘GLACIAL VARVED CLAY CONCRETIONS—BASSLER 
oid, annulated, lenticular, and cy- 
lindrical. He noted further that the 
concretions lie parallel to the bedding, 
are flattened vertically, are wider than 
high, that the upper surfaces are 
variously ornamented and in relief, 
often built upward by the superimpo- 
sition of many films, while the bottoms 
are apt to be flat and little ornamented. 
The lines of stratification were often 
noted to run unbroken through the 
concretion and its surrounding clay, 
indicating, first, its formation subse- 
quently to the deposition of the clay 
and, second, such quiet growth that 
the stratified layers were not disturbed. 
An important factor in explaining 
the concretion shapes has been found 
to be the varying amount of dissolved 
calcium carbonate in the different clay 
layers. Similar forms from the same 
bed are essentially alike in composition 
but symmetrical forms differ from un- 
symmetrical ones; furthermore, sec- 
ondary modifications in shape may be 
produced by mechanical pressure as 
shown by bent concretions, especially 
since some specimens have been re- 
corded as found in still plastic condi- 
dition. Long, narrow concretions 
probably arise from the coalescence of 
several examples resting in a straight 
line in the same layer. Weathered or 
broken specimens of the Ryegate con- 
cretions seldom reveal a visible nu- 
cleus, a few cases only being known 
where a pebble was found at the cen- 
ter. Many examples show that the 
concretionary process may be inter- 
rupted during its course and after- 
ward begin again, as shown by the 
changes in color of the component 
parts and also by the fact that such 
specimens often fall apart when 
weathered. 
It must be remembered that concre- 
tions, like other inanimate objects, 
grow by external additions, layer by 
layer. It is thus that they assume such 
curious and often fantastic forms. 
The shape and ornamentation of spec- 
imens in bedded deposits, especially 
thin-layered clays, are influenced by 
their position in the layers and the dif- 
273 
ferent porosities of the clay beds. Thin 
disks without ornament thus originate 
in the thin, porous layers bounded 
above and below by impervious layers. 
Thicker porous layers allow the forma- 
tion of more rotund specimens but 
smooth and flat on the bottom if 
growth starts next to a lower impervi- 
ous layer. 
The writer had no opportunity for 
field work in the study of these con- 
cretions, so that he welcomed the help 
of Harold M. Davis, of North Calais, 
Vt., in kindly collecting several dozen 
samples of Ryegate varved clay show- 
ing specimens still in place. Edge 
views of three of these samples are 
illustrated in plate 1, where the con- 
cretions are seen in the white porous, 
silty layers only, usually occupying 
their full thickness. All these samples 
show the same phenomenon, which 
suggests that this is their normal place 
of formation in this outcrop of the clay, 
at least. This white layer today 
quickly dissolves into a fine noncal- 
careous silty mud, while the dark lay- 
ers do so with difficulty. It would 
seem, therefore, that the remarks on a 
previous page, derived from views of 
various students on the formation of 
these varved clay concretions, are es- 
sentially correct. 
Concretions from other New Eng- 
land localities are described by Dr. 
W. A. Tarr in his excellent article 
*‘Concretions in the Champlain For- 
mation of the Connecticut River Val- 
ley,” published in volume 46, Bulletin 
Geological Society of America, 1935. 
Almost all of them also are restricted 
to the sandy or silt layers’: and none 
occur in the dark clay beds. The stu- 
dent is referred to Dr. Tarr’s work for 
accounts of the detailed chemistry of 
concretion origin and the factors con- 
trolling their form. 
The white and dark layers of the 
varves in ancient lakes of nonglaciated 
regions are explained by the changes 
of seasons as follows. The white layer 
is usually highly calcareous and the 
dark one charged with organic matter. 
In summertime, higher temperature of 
