123 
More recent studies, in connection with the Ohio Survey, 
had revealed to him a like series of conditions in the rocks of 
the Mississippi valley. 
This peculiar and suggestive succession he had shown to 
exist in each of the great Paleeozoic ages (Ohio Final Report, 
Vol. I, part 1, chap. ii.), as clearly as it is seen in the Creta- 
ceous of the Westand South-West. It can likewise be recog- 
nized, though less perfectly, in the Cretaceous of the Atlantic 
coast. The Trias of the far South-West also shows a like suc- 
cession, though with certain peculiarities of difference. In 
New Mexico it is beautifully displayed at many points, af- 
fording sections in some cases of over 2000 feet. At the 
base there is always a mass of sandstone and conglomerate 
(the source of the pyrope garnets and peridots, which are 
found washed out and distributed in the gravel of stream- 
beds, etc.). Then follow shales, marls, and occasionally 
magnesian limestones. ‘The succession is so precisely similar 
to that of the Triassic rocks of Europe, that Marcou recog- 
nized and named the group before he had procured from it a 
single fossil. (These several series were all discussed and 
tabulated at length.) 
The distribution of Mesozoic rocks abroad was then taken 
up and treated of in like manner. The facts of this kind of 
succession have for some time been familiar to geologists, but 
their origin has never been fully understood or presented, as 
the simple and inevitable consequence of great periodic in- 
vasions of the ocean. LHaton, years ago, had remarked the 
succession of three types of formations, calling them, in their 
order, “ silicious, caleareous, and carbonaceous series.” Profs. 
Dawson, Hall, and Hunt have recognized the same facts dis- 
tinctly ; while in Kurope, Murchison and his colaborers in 
Russia have described the Permian as haying a trinal charac- 
ter, similar to that of the Trias, which, as every student of 
the science knows, derives its name from this three-fold di- 
vision—sandstone, limestone and shale. More recently, Mr. 
Edward Hull has given extended tables of both the Palzeozo- 
ic and later rocks of England and the Continent, based on 
this system of grouping.* 
Of course, the operations of nature are everywhere compli- 
cated by countless minor circumstances, which tend to break 
the regularity and simplicity of such outlines. We find 
smaller cycles sometimes within the greater ones (as notably 
* Jour. Geol. Soc., London, Vol. XXVIII, p. 132. Geol. Mag., Vol. 
V, p. 148. Quart. Jour. Science, Vol. VI, p. 353. 
