GEOLOGY AND PALEONTOLOGY. 145 
Doctor Lesquereux, nearly thirty years ago, promulgated a theory to account 
for these beds, which to the mind of the writer is the most reasonable of any yet 
advanced. It will be given in a somewhat abbreviated form in the language of 
the noted author. After pointing out the resemblance of the Dakota to certain 
sandstones of Devonian and Carboniferous ages, Doctor Lesquereux says: 
‘The origin of these groups of sandstone is the same, to my belief at least. 
They are beach formations, like those in progress at the present time along the 
shores of the North Sea, in Holland and Belgium, where the widely extended, 
muddy shores are formed ‘of asoft substance of thesameredcolor. As I have had 
opportunity to examine it, it is a mixture of small grains of sand, brought from 
the sea with the mud deposits, carried by rivers of long course, after traversing 
flat countries. By slow deposition it constitutes low shores, successively washed 
by the tides, which of course recede or advance farther in proportion to the slow 
upheaval or depression of the land. Marine animals, the shells especially, are 
very rare in a formation of this kind.’’™ 
Then follows a short discussion of the leaves of the Dakota, from which the 
conclusion is drawn that the essential types of the vegetation are those of low 
islands and shores, rather than of hills and dry land. In speaking of the origin 
of the leaves, he remarks: 
a The Cretaceous leaves have been derived from trees or groups of 
trees growing in the vicinity of muddy bottoms, where they have been buried 
and fossilized. . . . The deposits seem to have been local, dependent upon 
circumstances. There must have been, necessarily, an arm of the sea, with soft, 
sandy mud, bordered by the adjoining dry land covered with a forest. The 
characteristics of the local deposits indicate that the forests were on small 
islands scattered over the Cretaceous sea.”’ 
oc . The formation of the Dakota group is the result of muddy flats 
whose surface, raised perhaps in hillocks above water limits, and already solid 
ground, was cut like an immense swamp, here and there interspersed by rare 
groups of trees and bushes. Of course the main portion of this surface was sub- 
ject to continuous change in the successive modifications resulting from the 
heaping and displacement of matter by water, and thus the leaves were dis- 
tributed either at the same place but at different levels, or at the same level but 
at different localities.”’ 
To this it seems necessary to add that recent investigations in the field but 
confirm the accuracy of the theory. 
PECULIAR FORMS OF STRATIGRAPHY. 
While the ordinary characters of Dakota stratigraphy are those described, 
yet throughout the group there are departures from the normal form that de- 
serve especial] notice. These peculiar forms may be confined to special localities, 
or, as in the case of nodular concretions, may be distributed throughout the en- 
tire group. These concretions vary greatly in form, size, and color. Oftentimes 
they are small and box-shaped, with angular corners and edges, an inch or two 
across, with a predominating dark-brown color. They break with a conchoidal 
fracture, exposing cutting edges. The enclosed cavity is usually filled with 
white or cream-colored clay. More frequently, perhaps, the concretions are 
spherical or geodic in structure, and when broken form hollow, bowl-shaped 
vessels. Not infrequently these lie scattered over the surface, forming reservoirs 
in which rain-water collects. Very often the concretions are solid, varying in. 
size from that of a pea to that of a baseball. These may be found either solitary 
or cemented into clusters. Sometimes these clusters will be grouped together, 
resembling a bunch of grapes; or perhaps but two will be joined, something 
after the manner of a double walnut. 
50. Cretaceous Flora, p. 27, et seq. 
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