Otlmicl Charles Marsh. — Becclicr. 143 
that PalccotrocJiis and Ligiiilites were not of organic origin, 
though the contrary had been previously supposed. 
In the domain of geology, his chief interests lay in the 
formations from which he secured important series of fossil 
vertebrates. Probably his greatest geological discovery was 
the Uinta basin, an Eocene deposit of the eastern Uinta 
mountains. It was first visited in 1870. Having studied most 
of the Tertiary lake basins in the Rocky Mountain Region, 
he gave, in 1875, a synopsis of their geological features. As a 
natural result of studying geology in Germany, he was much 
impressed with the methods of marking the separate horizons 
by means of some characteristic fossil. He believed the verte- 
brates were the most sensitive time-markers, and therefore 
endeavored to determine and limit geological horizons wholly 
by fossil vertebrate remains. The inherent fault of this sys- 
tem is that the vertebrates are not always the most highly 
differentiated and specialized types in any given fauna, and it is 
these qualities alone than can be safely employed in organic 
chronometry. This method is usually of great value in fresh- 
water deposits rich in vertebrate remains, but it can be seldom 
used to advantage in marine sediments or in formations con- 
taining a scanty vertebrate fauna. Thus, wh'ile the name 
Equus Beds is very appropriate for a horizon irf the Pliocene, 
on account of the abundance of remains of fossil horses, the 
same cannot be said of the term Eosaurus Beds as an equival- 
ent of the entire series of the Coal Measures, especially as but 
two vertebrae of this animal have ever been discovered. Geol- 
ogical facts will be found scattered through many of his publi- 
cations dealing principally with fossil vertebrates. One of the 
latest problems to interest him was the age of the series of 
variegated clays extending from Martha's Vineyard south 
along the Atlantic coast into Maryland. His investigations 
led him to refer them to the Jurassic, a formation which had 
been considered as absent in eastern North America. 
There yet remains for consideration the real work of his 
life, — his publications on the fossil vertebrates, and it is at 
once evident, from a glance at his bibliography, that his chief 
researches were upon the reptiles, birds, and mammals. There 
are three papers on fossil fishes, containing notices of several 
new forms, but no real research in this class was ever under- 
