124 
in the coal-measures), periods of rest or of regression in the 
course of these movements. Butthe great outlines are there, 
and may be clearly read by the observant eye and mind. 
The lowest member of each series is always a fragmental 
deposit, made by the wash of the ocean on the shore, and 
containing as fossils the remains of land-plants. The middle 
member is a limestone, with marine fossils, plainly the 
product of deeper and clearer waters. The series is closed 
by the deposits of a retreating and shallowing ocean, clays, 
shales, and mixed material, in which neither the coarse de- 
posits formed by the wear of an invading sea, nor the calca- 
reous organic sediments laid down by the clear ocean, could 
prevail. 
Pror. Wurtz suggested for the three types of formation, 
the names of Silicious, Caleareous, and Aluminous. He dwelt 
upon the importance of recognizing chemical action as one 
great agency in determining the character of deposits, and 
referred to his use of the terms “ ferric” and “ferrous,” to de- 
note certain alternate and oft-recurring periods in geology. 
The lowest members of each of these great cycles of deposi- 
tion are generally ferric, 1 e. contain iron in the state of 
sesquioxide, having been exposed, during their formation, to 
atmospheric oxidation. To this fact, as is well-known, is due 
their prevailing red color. In like manner, the upper mem- 
bers of the series are usually ferrous, having their iron in the 
protoxide state, from the deoxidizing agency of organic mat- 
ter present in the sediment when laid down. 
Tue PRESIDENT observed that Prof. Wurtz’s generaliza- 
tion as to the state of the iron is usually correct. He pre- 
ferred, however, to designate the three types of deposits as 
Mechanical, Organic, and Mixed. 
Pror. E. H. Day made some further remarks on the 
President’s paper, comparing its principles with the facts as 
illustrated in the Jurassic rocks of Europe; and referring to 
Prof. Huxley as having, in 1862, dwelt upon the similarity 
frequently observed in rocks of the same age, in different 
