C orresjjoiid enci' . 327 
Paleontologists might object to this statement and say that fossils 
furnish a sure guide to the contemporaneity of formations. But it must 
not be forgotten that fossils are not strictly limited in range in many 
cases; and even when individual species are limited the fauna as a whole 
is not. Facts in geographical distribution at the present time should 
teach the difficulty and precariousness of generalizations as to range in 
former times. The genus Sequoia is living now in America, but it 
lived in Europe during the Tertiary. Were the species to become extinct 
now, a correlation of deposits in the two continents and their reference 
to the same time period would be obviously an error. This is only one 
instance out of many that might be cited. 
The subject is still further considered by Prof. Lesley in his discussion 
of the separation of the Hudson River slate and Medina shales. In Bed- 
ford county the Oneida sandstone is absent and the Hudson River slates 
pass without any break into Medina shales. Hence the inference is 
drawn that as there is no indication of an elevation of the sea bottom to 
dry laud, the absence of the Oneida "is really and surely due to the fact 
that the sediments were floated further out into deep water according 
to their fineness, until at length the finest material was exhausted, or, 
mingled with equally fine material, floated in from other directions." 
(p. 665.) At the Lehigh water-gap, while the Oneida is present, it rests 
conformably upon the Hudson River sandy slates, and these uppermost 
beds contain thin beds of sandstone, and there is thus a regular passage 
from the fine beds of the Hudson River to the coarser beds (jf the Me- 
dina; "and yet the transition is in fact instantaneous; as if a vast 
quantity of gravel was deposited upon a level sea bottom of dark sandy 
mud." (p. 676.) 
So, too, at Port Jarvis, the Oneida and Medina pass into each other, 
by a series of alternations of white and red beds. "The bottom Medina 
beds (next over the Oneida) are all sandstone, made up of grains of 
quartz, some of them containing small pebbles of white quartz, inter- 
stratified with soft shales; while the upper Medina beds are nearly all 
reddish shale (much split by cross cleavage) interleaved with thick red 
and grayish sandstone beds" (p. 677). 
The student of topography will find in Chapter LII (pp. 681-711) an 
interesting discussion of the topographical features of middle Pennsyl- 
vania. This is a mountainous region and the chapter is a succinct his- 
tory of the mountains in their geological aspect. 
Volume two is occupied with the Upyjer Silurian and Devcmian for- 
mations as volume one was with the Lavu-entian, Cambrian and Lower 
Silurian. In Formation No. V, we find included the C'linton. Niagara 
and Salina of the New York system. The contacts of this with No. IV 
below and No. VI above are indefinite and difficult to locate. The gen- 
eral thickness of the formation is about t?()0 feet, but the local variations 
are very sudden and very great. The beds are mostly destitute of fossil 
remains. One remarkable feature is that the Niagara formation, so 
well developed in New York, can not be certainly recognized in Penn- 
