26 AGE OF THE GYPSUM FORMATION, 
which to base an opinion or conclusion respecting the part of the Carboniferous series to which 
these strata may be referred; but, whether upper or lower, the group, if Carboniferous, differs 
materially from the Carboniferous formation as developed further to the north and east. The 
same difficulty arises if we consider the Gypsum formation as Triassic; the same apparent defi- 
ciency of strata, well characterized at other localities, exists. 
There are many reasons for referring the formation to the age of the Cretaceous, for this is 
the only period represented by fossils; and from the wide extent of the strata, we are compelled 
to believe that they attain a very considerable thickness—a thickness greater than that of the 
light colored strata of the Llano above the red clay and gypsum. The whole series, from the 
surface of the Llano, of Cretaceous age, down to the underlying Carboniferous sandstones, might 
be regarded as Cretaceous. If we admit the mineral characters of the strata as evidence of the 
age of the formation, they will favor the conclusion that it is Carboniferous. 
Dr. D. D. Owen (as has already been noted, page 18,) reports the existence of great beds of 
gypsum in the Carboniferous shales of the coal-basin of Iowa; the beds being from twenty to 
thirty feet thick. So, also, the great beds of Nova Scotia, which supply such large quantities of 
massive gypsum and alabaster for exportation, are in the Carboniferous formation, and not in 
the new red sandstone, or **Trias." Mr. Dawson, in his recent able and interesting work, enti- 
tled **Acadian Geology,’’ gives detailed descriptions of these beds, and of their association with 
the marls, sandstones, and limestones of the coal period. "The well-known gypsum-beds of 
Windsor, Nova Scotia, are found in connexion with a limestone containing fossils of the lower 
Carboniferous formation, of the genera Productus, Spirifer, and Terebratula. Mr. Dawson 
makes the following observations in regard to the position of the gypsum: ‘‘ Above this lime- 
stone, in the order of succession, we have alternations of marls and limestones, and next a 
bed of white crystalline gypsum, contrasting strongly in its purity and whiteness with the 
other beds of mere mechanical origin. Here the shore becomes low, and no rock is seen; but 
a little to the eastward we find the great gypsum quarries of Windsor, excavated in the out- 
crop of a very thick bed, the strike of which would bring it to the shore just where our section 
fails, and where the gypsum has been removed, partly by the river, and partly by the quarry- 
men who earliest dug this rock for exportation. A little further to the southward, at the next 
bluff point, there is a very thick bed of limestone, filled with, or rather made up of, fossil shells 
of various species and genera, affording a remarkably perfect display of the shelly coverings of 
creatures that inhabited the Carboniferous seas.’’! The following species are enumerated, and 
figures given: Productus Lyelli, Fenestella membranacea, Terebratula elongata, Ceriopora spon- 
gites, and new species of Orthoceras, Spirifer, and Conularia. The bivalve shells are very 
numerous, especially four or five species of Terebratula, three of Spirifer, and three of Produc- 
tus. The Carboniferous age of this Gypsum deposit is thus established; and I have been par- 
ticular to present the evidence upon which the conclusion is based, as Mr. Jules Marcou, who 
regards the red marls and gypsum of New Mexico as Triassic, has affirmed their synchronism, 
or connexion with the Nova Scotia deposits of Windsor and Plaister Cove.? 
I do not regard the physical and mineral characters of a formation as sufficient to establish 
its geological age, or relative position in the scale of paleozoic formations. In the absence of 
fossils, I do not, therefore, assign the age of the Gypsum formation; but its relative position, 
and its mineral characters, unite in showing it to be Carboniferous, or Cretaceous, rather than 
Triassic. 
Fort Belknap to Preston.— According to Dr. Shumard,? the geological formations in the 
vicinity of Fort Belknap consist of nearly horizontal strata of fine-grained sandstone, shale, 
and soft, drab-colored, non-fossiliferous limestone, whose relative positions correspond with 
! Acadian Geology. By John William Dawson: Edinburgh, 1855, p. 218. 
Bes ney of Explorations for a Railway Route, &c., by Lieutenant A. W. Whipple. House Doc. 129: Washington, 1854, 
* "ed of Captain Marcy of Exploration of the Red River in 1852. —Appendix D, p 182. 
