iSSg.] Description of the Devonian Rocks of Iowa. 229 
vvojld not really exist; on the contrary, the structural elements 
in the crystallized salt would be of two sorts— namely, R and 
3H,0."' 
Hence it is geometrically possible that the structural ele- 
ments of a crystal may be different atomic groups which are 
held in a position of stable equilibrium by virtue of being in- 
terpenetrating networks. 
A GENERAL PRELIMINARY DESCRIPTION OF THE 
DEVONIAN ROCKS OF IOWA ; WHICH CONSTITUTE 
A TYPICAL SECTION OF THE DEVONIAN 
FORMATION OF THE INTERIOR 
CONTINENTAL AREA OF 
NORTH AMERICA. 
BY CLEMENT L. WEBSTER. 
The area of the Devonian rocks in North America presents at 
least four distinct types of stratigraphy jn their sections, in differ- 
ent parts of the continent. 
The four types blend, more or less, at their borders, but in their 
central area are quite distinct. 
The four areas may be called, — 
(i) <' The Eastern Border Area;' including the outcrops of 
Gaspe, New Brunswick, Maine, and other places in Northern New 
England. 
(2) " The Eastern Continental Area," including the New York 
and Appalachian tracts as far South as West Virginia, and extending 
Northwestward into Canada West and Michigan. 
(3) " The Interior Continental Area,'' typically seen in Iowa, and 
extending into Missouri, Illinois, Indiana, and probably Northward 
toward the valley of the Mackenzie River, and— 
(4) " The Western Continental Area;' best known through 
Hague and Walcott's studies of the Eureka, Nevada, sections.^ 
Each of these four types presents sections of the Devonian, which 
J Sohncke, Zeitsch. /. Kryst. xiv. p. 443. 
'■ This classification of (in part) Professor H. S. Williams (American Geologist, 
Special Number, October, 1S88, p. 228) we here adopt, provisionally. 
