GEOLOGY: /. BARRELL 
501 
of Lingula, Orthoceras, and Beyrichia. With these are found abun- 
dantly at several levels waterworn scales and plates of ostracoderm 
fishes. The sandstones are succeeded by marine limestones of Trenton 
age, but in the limestones, although marine fossils are abundant, the 
fish remains are absent. If we seek to interpret the origin of these sand- 
stones and their fish remains, it is to be noted that the thickness, 86 
feet, is large to have been formed by wave action on a nearly base 
leveled land. But rivers bring most of the sediment to the sea which is 
laid down as sandstone, and it is probable that most of this sand was of 
river derivation. The waterworn character of the fish remains indicates 
a degree of transportation which the other fossils do not show, and sug- 
gests driftage into the region of burial. The absence of all fish remains 
in bothOrdovician and Silurian deposits of solely marine nature strength- 
ens the view that these rare and fragmentary finds were floated into the 
marginal waters from the rivers. 
In mid Silurian red shales of Pennsylvania and in the Upper Silurian 
Ludlow rocks of Great Britain have been found the next clear appearance 
of fish remains, mostly of ostracoderms, in associations which seem to in- 
dicate at this geologic stage a life of these bottom dwellers not exclu- 
sively in the rivers but also probably in protected and brackish em- 
bayments of the sea. More clearly, however, they did not yet live in 
the open sea. 
The first true fish fauna, as represented by sharks, in better preser- 
vation and considerable variety, is found in the Lower Old Red Sand- 
stones of the Caledonian basin. These are lowest Devonian in age. 
But the sediments are of a different nature from the marine or brackish 
water deposits of the Ordovician and Silurian which held the earlier 
fossils. Viewed in the light of the modern knowledge of the nature of 
alluvial deposits, the sediments of the Old E.ed Sandstone are inter- 
preted as having accumulated as river deposits on flood plains or in 
shallow lakes of an interior basin, lakes subject to marked shrinkage in 
area during the dry season. 
The ganoid fishes appear for the first time and in force in the Middle 
Old Red Sandstone formations of the Orcadian basin of northeastern 
Scotland, twenty species belonging to seven genera being known from 
that basin. Dipnoans also appear at the same time. The enveloping 
deposits are regarded as Lower Devonian in age, though younger than 
the deposits of the Caledonian basin. 
In contrast to this land water record of the lowest Devonian, the 
sharks are not found in declaredly marine rocks until at least the Middle 
