SEDIMENTARY ROCKS 197 



stitute a much smaller proportion of the formation than the shales, and 

 occur prevailingly in beds less than one inch thick, but are in places as 

 much as 3 inches or even more in thickness. The limestones characteris- 

 tically occur in beds having a thickness of less than one foot. 

 ■ The Carboniferous invertebrate remains from these shale beds along 

 the boundary, as well as from some of the upper members of the Racquet 

 group, were sent to Dr. George H. Girty, of the United States Geological 

 Survey, who has reported on them, and has assigned them all to the same 

 horizon, showing that they are stratigraphical equivalents, as described in 

 more detail further on in this paper. Doctor Girty reports concerning 

 these fossils as follows: 



"This group of collections contains types so similar to Russian species de- 

 scribed by Tschernyschew in his monograph on Gschelian Brachiopoda that it 

 sieems almost inevitable to correlate them at least provisionally with the 

 Gschelian stage, which occurs just below the Lower Permian (Artinskian) of 

 the Russian section. This is true of both groups of collections, though they 

 show fairly distinct fades from one another, for both are about equally related 

 to the Gschelian, yet herein enters an element of doubt, owing to the singular 

 fact that among Tschernyschew's Gschelian brachiopoda, and, indeed, in the 

 associated fauna, are numerous species which not only lack corresponding types 

 in our own Pennsylvanian, but are closely related to types which seem to be 

 restricted to the Mississippian. Some of the collections contain few, if any, 

 of these types, whereas others contain a considerable number of them, and the 

 question Is Immediately raised whether we are to rely upon the one set of 

 affinities and call the horizon Gschelian or on the other and call the horizon 

 Mississippian. Since, however, we have in Alaska a Lower Carboniferous 

 horizon equivalent to the Productus giganteus zone of Europe, to which the 

 present fauna does not appear to be closely related, it seems more probable all 

 should be correlated with the Gschelian. 



"I should perhaps add that in the case of all these identifications and corre- 

 lations I have met not only the usual difiiculty, that many of the fossils are 

 poorly preserved so that their identification, and consequently their signifi- 

 cance, is doubtful, but also the additional one that I have been entirely without 

 specimens of the Gschelian fauna with which to make comparison and have 

 had to rely solely upon descriptions and figures, chiefly the latter, as the text 

 is mostly in Russian." 



Racquet group. — The members of the Racquet group are nowhere very 

 extensively developed within the belt along the Alaska- Yukon boundary, 

 which is here being considered. In the northern portion of the area, 

 however, they occur associated with the older limestones and dolomites, 

 and are in places difficult to distinguish lithologically from the limestones 

 of the Devonian or even earlier Paleozoic periods. This series has an ag- 

 gregate thickness of at least 1,500 feet, and consists mainly of limestones 

 and cherts, but includes also occasional beds of shale, calcareous sandstone, 



