Association of American Geologists and Naturalists. 157 
dies, and therefore proposed that the specimens presented by Mr. 
Brown be given to Mr. Silliman for this purpose, which was 
passed. 
Mr. James Hall read a communication ‘on the geographical 
distribution of fossils in the older rocks of the United States.” 
Mr. H. commenced with some general views as to the formation of 
sedimentary rocks. They required for their deposition materials to fur- 
nish the detritus sand or mud of which they were made up ; bodies of 
water in which these materials could be suspended, and from which they 
might be deposited according to their density—sand first, and the more 
finely comminuted particles forming mud afterwards. As to the ma- 
rine exuviee they contained, these would depend, as to distribution, num- 
ber and character, materially upon the depth of the ocean, the distance 
from the land or shore at which the matter enveloping them was depos- 
ited, and consequently where they had lived, and the nature of the bot- 
tom upon which they had their existence. 
In connection with this he might be allowed to mention, that Mr. Da- 
na had informed him that the forthcoming reports of the Exploring Ex- 
pedition would contain numerous facts as to the distribution of shells 
and crustacesze at the present day. He had met with no essay upon 
this subject, and the few scattered facts which were to be found in dif- 
ferent authors rather stimulated than satisfied curiosity. The few facts 
upon the subject which had fallen under his observation, were now of- 
fered, in the hope of calling the attention of others to the subject, rath- 
er than with the expectation of furnishing any complete solution of the 
problem. 
The rocks which in England are called Silurian, and which in this 
state we have termed the New York system, [under this name are com- 
prehended ihe Cambrian, Silurian and Devonian systems, which are 
now considered as forming one system,] are known to be of great ex- 
tent in this country. The researches of Murchison in Europe show 
their extent in that continent, and some of their fossil characteristics are 
known to occur abundantly in Siberia. The perfect development, wide 
range and comparatively undisturbed state of these rocks in the United - 
States, afford excellent opportunities for studying the condition of the 
ancient ocean, from which they were deposited over wide areas. He 
exhibited a section extending from the eastern part of New York to the 
Mississippi, in which he had endeavored to point out the comparative 
developments of the different strata. Having travelled over this ground, 
he was acquainted with the lithological and fossil characters of the rocks. 
He first considered the changes in lithological character which these 
strata exhibit in proceeding westwardly, and their greater or less devel- 
opment. 
