274 
the lake water caused abundant cal- 
cium carbonate precipitation and an 
increased quantity of floating micro- 
scopic organisms. The heavy carbon- 
ate sank to the bottom, forming the 
white layer. Later subsidence of the 
dark organic material, usually in the 
wintertime, resulted in the dark band 
of the varve. 
Most concretions are collected more 
for their unique shapes than for their 
geologic interest. It is rare to find 
specimens in collectors’ cabinets la- 
beled with any information other than 
their geographic locality, if even that 
be given. The exact geologic forma- 
tion and other information of occur- 
rence in the field must accompany all 
examples if they are to be useful in 
scientific study. The variety of forms 
they assume, their different composi- 
tions, their occurrence in most all 
types of rocks, and the conditions of 
origin whether primary or secondary, 
are all factors which, if carefully 
studied, may lead to important scien- 
tific results. To the economic geolo- 
gist concretions should be of interest 
as a source of minerals or ores brought 
together through their segregation 
properties. Many of them in certain 
sedimentary rocks have formed about 
well-preserved fossils making them of 
special interest to the paleontologist. 
The strange and often bizarre designs 
assumed by the varved clay examples 
should be an inspiration to the artistic 
world. 
The experience of collectors has 
shown that individual beds or portions 
of outcrops develop almost identical 
forms, just as the individual strata of 
sedimentary rocks carry their own 
characteristic fossils by which they can 
be identified from place to place. 
However, in the case of concretions this 
similarity is due to their identical con- 
ditions of origin and does not depend 
upon time differences as in the fossils. 
For example, the National Museum 
has an extensive collection of Croton 
Point, N. Y., concretions from sandy 
beds above the varved clays, which 
shows no symmetrical specimens and 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
an almost complete absence of the 
characteristic flat forms. Most of these 
specimens are greatly distorted, but in 
spite of this some indicate an attempt 
to assume the symmetrical patterns of 
the varved clay examples. 
In the present paper the writer has 
studied this exceptionally complete lot 
of concretions as if it were a collection 
of fossils from some specific geological 
horizon and locality, in this case the 
Post-Wisconsin (Champlain) varved 
clay of New England. The photo- 
graphs were prepared to illustrate the 
trends of concretion growth and the 
individual stages of each, in place of 
the genera and species of the organic 
world. ‘The plates, therefore, can be 
used for correlative purposes when 
studies of concretions from other 
geological formations and of different 
origins are undertaken. 
In spite of their diverse forms these 
specimens can be roughly classified 
into the following groups, omitting 
some irregular examples noted in the 
plate explanations. This outline dif- 
fers from Professor Hitchcock’s classi- 
fication of a century ago in employing 
the evolution of their shapes as a 
basis, but it is evident that any classi- 
fication of such variable objects is 
only a matter of personal opinion. 
Generalized Classification of Ryegate 
Glacial Varved Clay Concretions 
1. Simple, bulletlike forms growing into 
loesspiippchen (pl. 2). 
2. Thin, flat, smooth disks with faint cir- 
cular ornament, occurring in narrow 
layers of pervious clay (pl. 3, figs. 1-4). 
. Coin-shaped disks with smooth base and 
flattened confluent lobes on upper side 
(pl. 3, figs. 511). 
4. Hemispherical, smooth specimens in 
beds thick enough to allow growth in all 
directions (pl. 3, figs. 12, 13). 
5. Thin disks becoming confluent laterally 
with circular ornament on both sides 
and small lobes on upper (pl. 3, figs. 
14-19). 
6. Bulletlike forms uniting and developing 
into flattened dumbell shapes (pl. 4). 
7. Grapelike or botryoidal growths uniting 
into bulbous, twisted structures simu- 
lating animal and other forms (pl. 5). 
2 
