98 The American Geologist. February, 1894 
presence of one or more of these organisms. Deductions, for 
example, have been made upon Scolithus linearis, the presence 
of this fossil having- caused Billings* to place strata on the 
Strait of Belle Isle, now recognized as Lower Cambrian, in the 
same horizon as the Potsdam of New York, now in the Upper 
Cambrian. So the Chilhowee of Tennessee has heen placed in 
the Potsdam upon the evidence of the same fossil, whereas it 
is really Lower Cambrian. \ II. M. Ami, in 1 887 ^ when dis- 
cussing the occurrence of Scolithus canadensis in the Chazy 
of Canada said "that the occurrence of Scolithus remains does 
not necessarily indicate the presence of Potsdam rocks, but 
that the beds may possibly be newer or higher up in the 
series." The same fact was mentioned by Brainerd and Seely§ 
when discussing the rocks of the Champlain lake region. 
They stated that "the fucoids, so far as we have seen, are not 
characteristic of any one division, though they appear abun- 
dantly in various horizons of I). Further, Scolithus can not 
be regarded as indicating a Potsdam horizon, as the most 
abundant display we have ever seen is to be found at the bot- 
tom of Division (', 600 or 700 feet above the Potsdam sand- 
stone." Professor Lesley quotes this statement and agrees 
with it. || But he had come to the same conclusion fifteen 
years before. In a discussion of fucoids in that year,** he 
called attention to their wide time range and said: 
"There may have been different species, but certainly most of the ma- 
rine Alga? had a large vertical time-range through the Paheozoic for- 
mations. 
"Consequently they can not be used as geological guides. We know- 
nothing about their internal structure. They were cellular masses, 
easily rotted, rapidly losing shape when dead, and leaving on the an- 
cient shore-sand mere molds or indistinct impressions. They cover, by 
millions, the Chemung rocks (Oil system), and have a thousand shapes 
which seem at first to be easily classified into species and genera; but 
the task is fruitless; form graduates into form, and no specific marks 
*Billings, E. Pakeozoic Fossils, 1861, p. 2. 
tWalcott, C. D. 
JOn the occurrence of Scolitlms in rocks of the Chazy formation 
about Ottawa, Canada. Canad. Rec. Sci., vol. u, p. 304. 
§Bull. Geol. Soc. Am., vol.i, 1890, pp. 501-511. 
|| Dictionary of Fossils of Penn. PI, vol. in, p. 943. 
**On a map and profile of Coal and Oil Measures along Slippery creek 
in Lawrence county, Pennsylvania; from a Survey in 1864 by J. P. Les- 
ley and Leo Lesquereux. '2d Geol. Sur. Penn., J, 1875, pp. 90-107. 
