370 INDEX TO THE STEATIGRAPHY OF NORTH AMERICA. 



everywhere within a few feet of the contact of the conglomerate with the granite and uniformly 

 containing fossils, but found none farther south. The coarse sandstone and conglomerate 

 series in which this fossiliferous zone occurs appears to be stratigraphically continuous with 

 the Fountain formation in the Boulder district as defined by Dr. Fenneman," who correlates 

 it with the Fountain of the Colorado Springs region. We have traced the formation in the 

 field the entire distance from the Wyoming line to Boulder and find no reason to doubt that the 

 horizon containing the chert fossils is the equivalent of the base of the Fountain as represented 

 at Boulder, or possibly even much higher than the base as it occurs immediately north of Boulder, 

 where the formation is much thicker than near the Wyoming line. 



In limestones at a much higher horizon we found a Pennsylvanian fauna (tentatively con- 

 sidered older than Knight's so-called "Permian" of Wyoming), consisting of Produdus cora 

 D'Orb., P. nehrascensis Owen, Spirifer rockymontana Marcou, Squamularia perplexa McCh., 

 Derbya n. sp. ?, PMUipsia aff. major Shumard, Myalina swallowi McC, abundant crinoid stems 

 and others. These limestones were said by Darton * to pass into the Fountain conglomerates 

 in traveling southward. Of this we are by no means certain without further investigation, 

 but they seem certainly to belong at the very top of the Fountain or base of the overlying 

 Lyons formation of Fenneman's bulletin. These limestones nearly disappear at the Cache la 

 Poudre, and a little work just north of that stream is necessary to determine their exact posi- 

 tion. We have traced the contact of the Fountain and Lyons all the way from the northern 

 fine of the State to Boulder except 2 or 3 miles between the Cache la Poudre and Owl Canyon, 

 and believe the Tensleep sandstone of Darton's paper to be the stratigraphic equivalent of 

 Fenneman's Lyons. 



It seems very clear that all of the Fountain sandstones and conglomerates of northern 

 Colorado are Carboniferous, the base being probably Mississippian (certainly so in the northern 

 portion of the field) and the upper portion possibly as late as Pennsylvanian. It also seems 

 hkely that the Lyons is Pennsylvanian. 



The fossils were, through the kindness of Dr. T. W. Stanton, of the United States Geological 

 Survey, submitted to Dr. G. H. Girty, whose determinations are followed in this paper. 



Girty'^ states that he beheves the fossiliferous cherts which occur in the Foun- 

 tain formation to be derived from Mississippian hmestone, and he does not think 

 they determine .the age of the Fountain. David White" states that the fossil plants 

 found in the Fountain formation are of Pottsville age. 



The San Juan Mountain district of southwestern Colorado has become through 

 the studies of Cross, Spencer, Howe, and others, a standard of reference for the 

 geologic section of that central region of the Rocky Mountains where the Carboni- 

 ferous is characterized by several very distinct formations and unconformities. 

 The base of the section, the Ouray limestone, has already been described (pp. 

 367-368). The lower part contains a Devonian fauna, but an upper fossiliferous 

 horizon is of Mississippian age. Cross ^^* states : 



The erosion in the interval following Mississippian sedimentation removed those strata 

 completely over large areas. As neither the upper Devonian nor the basal Carboniferous horizon 

 is everywhere fossihferous, it is impossible to decide in many places whether the upper portion 

 of the Ouray limestone ledge is Carboniferous or not. 



From the great quantity of Carboniferous chert nodules in the succeeding formation it 

 must be assumed that above the known horizons of the Ouray there once existed in this region 

 a considerable thickness of chert-bearing limestone of Mississippian age. It seems not unUkely 



o Fenneman, N. M., Geology of the Boulder district, Colorado: Bull. V. S. Geol. Survey No. 265, 1905. 

 b Darton, N. H., Geology and underground water resources of the central Great Plains: Prof. Paper U. S. Geol. 

 Survey No. 32, 1905. 



'^ Notes on the manuscript of this paper. 



