PYCNODESMA, A NEW MOLLUSCAN GENUS FROM THE 

 SILURIAN OF ALASKA 



By Edwin Kjrk 



Of the United States Geological Survey 



Widely distributed in Alaska is a series of predominantly cal- 

 careous sediments that seems to represent a high upper Silurian hori- 

 zon. It is known with certainty in the Seward Peninsula and in 

 southeastern Alaska and probably is present in the upper Yukon 

 Valley in the Fairbanks region. Fossils from this horizon have at 

 various times been referred to the Devonian, Silurian, and even the 

 Mesozoic, the latter tentative determination being made on the evi- 

 dence of huge fragmentary pelecypods referable to Pycnodesma^ the 

 genus described in this paper. 



The best section of this part of the upper Silurian in southeastern 

 Alaska is on the south shore of Freshwater Bay, Chichagof Island. 

 Here Middle Devonian rocks are found at each end of the section, 

 and although it can not be told with certainty it appears that both 

 contacts are due to faulting. Some miles northeast of this exposure, 

 between False Bay and lyoukeen Cove, is another faulted block of 

 the upper portion of the series, bounded on the north by what is 

 taken to be Middle Devonian and on the south by Mississippian. In 

 Glacier Bay the isolated exposures of Willoughby and Drake Islands 

 furnish the best collecting grounds for the fossils of the lower part 

 of the series. The limestone appears elsewhere in the bay but as a 

 rule is considerably metamorphosed. On the south shore of Kosci- 

 usko Island, off the west coast of Prince of Wales Island, Pycnodesma 

 was collected but without the associated forms found elsewhere. It 

 probably represents a somewhat earlier horizon than is present at 

 the localities noted above. 



This horizon represents the uppermost stratigraphic unit of the 

 Silurian in southeastern Alaska and consists of approximately 3,000 

 feet of calcareous sediments. The basal portion of this series con- 

 sists of some 1,200 feet of massive limestone, followed by 1,800 feet 

 or more of argillaceous limestone with an occasional intercalated 

 layer of greenstone. So far as known this series is underlain by 



No. 2692.— Proceedings U.S. National Museum, Vol. 71. Art. 20 



48190 — 27 1 1 



