•^26 Tht Am erica n GeohKjiNt. Novcmbor, 1H9(> 
the Hui'onian system merely because it underlies the Potsdam sand- 
stone and suggests the Marquette ores; especially in the face of the fact 
that Marquette iron ores are not represented in the section along lake 
Huron, nor do they immediately underlie the Potsdam sandstone on 
lake Superior." (p. 158.) Similar remarks are made on page 162. 
As regards the Potsdam sandstone or Formation No. I, Prof. Lesley 
considers it to be unjjroved that the real Potsdam of New York extends 
as far as southern Pennsylvania, and he believes that the (Viiques sand- 
>ftone. is the best name for the Pennsylvanian outcrop. It is the Primal 
sandstone of Rogers and is more of a quartzite than a sandstone. Sco- 
litlivH lineoris is the only fossil mentioned by Prof. Lesley, although 
others have since been found. 
Following the discussion of the economic features of Formation No. 
I, mainly iron mines, is an interesting chapter on the Great Valley 
{Chap. XXII), and then is taken up the description of Formation No. 
II, the equivalent of the Calciferous, Chazy and Trenton of New York. 
It would occupy too much space and extend this notice to an unrea- 
sonable length to note the many interesting facts presented by Prof. 
Lesley in his graphic style. Some things, however, seem worthy of 
special notice. In discussing, for example, the thickness of Formation 
No. IV, including the Oneida and Medina of the New York series, he 
notes the lack of uniformity in all oceanic deposits and enforces his 
statement with the following remarks: 
"The bottom of a formation in one place may not exactly correspond 
to its bottom in another place; and the same is true of its top. Nature 
has never written its historical memoir of geological operations in dis- 
tinct and well-rounded sentences; has never numbered and headed its 
chapters; has seldom drawn strong black lines between its paragraphs. 
The formations grade and fade away into each other: and that both 
downward and upward: and the geologist who attempts to measure any 
formation at any place must simply do his best to select some bottom 
rock to begin it with and some top rock to end it with. But in doing 
this he is always liable to mistake. He must make his own selections 
on his own responsibility. He can never confidently assert that the bot- 
tom and the top of his formations are established facts of science. 
When he multiplies his measurements of a formation in various places 
in order to obtain by comparison a knowledge of its variations in thick- 
ness, he subjects himself to the risk of multiplying his errors. Some- 
times, indeed, a special bed at the bottom or at the top of a formation 
is so flagrantly different in constitution, in color, or in its fossil forms, 
from all the other beds near it that he can adopt it as a key rock with 
considerable confidence. But this is rarely the case; and even when 
such a key rock presents itself in one part of his district and another 
such key rock almost or exactly like it pres?nts itself in another part of 
his district there is always a possibility that the two are not continuous; 
that they were not deposited at exactly the same time throughout the 
region; and that perhaps nature has repeated the deposit locally and 
subsequently." (p. G29.) 
