70 PENNSYLVANIA AND NEW YORK DEVONIAN. [bull. 120. 
CONCLUSIONS. 
This study of the Devonian system in eastern Pennsylvania and New 
York offers an opportunity to compare the different series of rocks 
composing the system in that region with those of the more typical 
sections in central and western New York. A comparatively small 
amount of investigation has latterly been given to the rocks of this 
system in eastern New York, while their continuation in eastern and 
central Pennsylvania has received much attention. On this account 
it seems better at present to discuss more particularly the conclusions 
of the Pennsylvania survey. In reference to the work of the Penn- 
sylvania geological survey it is but just to state that the complicated 
geologic structure of the State has been worked out in an admirable 
manner and the stratigrapkic position of the various formations accu- 
rately determined, so that the State has reason t.o be proud of its most 
excellent geological atlas. But with the exception of the fossil plants 
of the carboniferous, the paleontology of the State has never received 
the attention it deserves. This failure to give paleontology its due 
consideration is evident in the correlations of the local formations of 
the Paleozoic rocks of the State with the typical sections of these 
formations in New York. If the writer be able to correlate more accu- 
rately a part of the Devonian system of Pennsylvania with rocks of 
the same age in New York, he realizes that it is because he has had 
the advantage of a thorough field study of these formations at their 
classic localities, supplemented by careful laboratory study of their 
fossils. The study of the New Yorlf sections and fossils was under 
the guidance of Dr. Henry S. Williams, then at Cornell University, 
who has added so much to our accurate knowledge of the Devonian 
system of the United States, and to whom the writer is greatly 
indebted for methods and suggestions in working out this problem. 
The Marcellus shale at the base of the Middle Devonian is generally 
clearly defined, except in the upper part, where it changes rather grad- 
ually from the argillaceous to the more arenaceous shales of the over- 
lying Hamilton stage. The presence of fossils usually indicates the 
stage to which the shales belong, although in one or two localities 
species generally regarded as restricted to the Marcellus were found 
associated with those of the Hamilton. Prof. Lesley has called atten- 
tion to this gradual transition from the black Marcellus to the over- 
lying Hamilton shales, and says " it is evident that the distinc- 
tion between Marcellus and Hamilton formations is a purely arbitrary 
one, the Marcellus deposits being continued into the Hamilton age; 
no reason but their darker color justifying their name; as the deposit 
became coarser its color growing lighter, chiefly, in all probability, 
because fine or mud clay has the faculty of absorbing and retaining 
petroleum better than coarser or more sandy clay." l Still there seems 
as good reason for retaining the Marcellus stage as that of many other 
1 Sum. Desc, Geol. Penu., Yo\, II, f. n., pp. 1204, 1205. 
