122 CARBONIFEEOUS FORMATIONS AND FAUNAS OF COLORADO. 



SAN JUAN REGION. 



In l!)00 appeared a careful description by Cross and Spencer of the geology of 

 tlie Rico Mountains," tlie same group which in the llayden atlas is set down as the 

 Boar River Mountains. The strata described are classified as Devonian, Carbonifer- 

 ous, .luratrias. and Cretaceous. The Devonian is reported as consisting of three 

 members. The lowest is a sandstone or qiiartzite, frequently conglomeratic in the 

 lower part; the middle is a shale, and the upper a limestone, for which the name 

 Ouraj' limestone is emplo3'ed. It is said that the same series occurs in the Animas 

 region, and apparently some points in the description are amplified from this wider 

 knowledge. '"In certain localities in the San Juan the quartzite was not deposited 

 upon the Algonkian, and elsewhere both the quartzite and the shale are missing, in 

 which case the limestone I'ests directly upon the pre-Paleozoic; but whenever the 

 lowest member is present it is always followed in order by the other two in conform- 

 able sequence. It is possible that the shale and quartzite are pre-Devonian." There 

 are thus indications of a species of unconformity between the several portions of the 

 Devonian beds. 



The quartzite is again described as massive, very dense, and highly indurated, 

 in color dull j-ellow and white, with red and brown staining. Its thickness is given 

 as about 200 feet. At the time this report was written no fossils were known from 

 the quartzite, and its reference to the Devonian, as shown in the statement quoted 

 above, was clearly made in a tentative manner. The character and geologic position 

 of this bed are so similar to those of the Cambrian of other parts of Colorado as to 

 suggest the Cambrian age of this series also. Furthermore, in some talus, with little 

 doubt derived from this quartzitic formation, Mr. Cross has recently found a slab 

 containing numerous specimens of a linguloid shell, probably belonging to the genus 

 Oholus {Lingulella). This evidence, though imperfect in several ways, combined with 

 the other, makes the Cambrian age of the quartzite member highly probable. This 

 formation Cross now proposes to call the Ignacio quartzite. 



The central shaly member of the Devonian series is not specifically described, 

 but its thickness appears to be about 50 feet. If the quartzite does, as I suspect, 

 represent the Sa watch quartzite, the Yule limestone, as such, has no equivalent in 

 the Rico section. The relation of the shaly member to the series established for the 

 Crested Butte quadrangle is of course altogether speciilative, but the suggestion is 

 made that it may correspond to the shale which in that section Spurr correlates with 

 the Parting quartzite. This correlation is rendered somewhat the more probable 

 because from this horizon Endlich * reports the occurrence of bones and scales of 

 fishes of large size, a type of fossil that seems to characterize the Parting quartzite 

 horizon. 



all. S. Geol. Sury., Twenty-first Ann. Rept., pt. 2, 1900, pp. 7-165. 



6 0. S. Geol. Geog. Surv. Terr., [Eighth] Ann. Kept., for 1874, 1876, p. 212. 



