290 BROOKS ANB KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



The heavy limestones and associated rocks of the lo^Yest group may meas- 

 ure several thousand feet. It is probably safe to estimate that the expo- 

 sures along the upper Yukon of Middle Devonian limestones and asso- 

 ciated greenstones indicate a thickness of from 5,000 to 8,000 feet, much 

 the larger part of which is made up of igneous rocks. The upper argil- 

 lites and associated sandstones are roughly estimated to have a thickness 

 of about 1,000 feet. 



The Devonian fauna of Alaska seems, so far as we know it, to conform 

 to a general and widespread tendency of the Devonian fauna to develop 

 provincial t3rpes. Neither in Alaska nor elsewhere do we know any type 

 of Devonian faima with such a worldwide distribution as the Middle 

 Silurian fauna, with which the Porcupine Silurian fauna has been corre- 

 lated in preceding pages. Very diverse types of faunas were developed 

 during the Devonian period, which sometimes flourished even in such 

 close proximity as contiguoiis portions of the same sea. It is not sur- 

 prising, therefore, to find strong contrasts between the fauna of the 

 Devonian limestone of the Yukon section and that representing a similar 

 horizon in soiitheastern Alaska. The latter also appears to bear little 

 resemblance in its composition to the best known of the Eocky Mountain 

 Devonian faunas, the fauna of Nevada,®^ or the Jefferson^® limestones. 



The marine basin represented by these limestones appears to have had 

 a considerable north and south extension, reaching from central Xevada 

 into western Montana ; but it was probably not connected with the south- 

 eastern Alaskan basin, since the Middle Devonian fauna of the latter 

 region appears to have a closer relationship to the Devonian of the Mani- 

 toba than that of the Eocky Mountain region. 



The southeast Alaskan and the Manitoba faunas contain foreign ele- 

 ments which are unlcnown in Devonian faiinas of north or eastern Amer- 

 ica. The presence in southeast Alaska of HercynelW and in Manitoba 

 of Stringocephalus,^'^ genera which are Avell known in Europe, but un- 

 known in eastern America, indicates both the relationship of these west- 

 ern faunas to those of Europe and a probable northwesterly route of inter- 

 migration between the Devonian faunas of western America and Europe. 



! 



88 C. D. Walcott : Paleontology of the Eureka district. Monograph no. riii, U. S. Geo- 

 logical Survey, 1884, pp. 99-211, pis. 2-6, 1.3-18. 



^ B. M. Kindle : The fauna and stratigraphy of the Jefferson limestone in the north- 

 ern Rocky mountains. Bulletin no. — , V. S. Geological Survey, 1908. (In prepara- 

 tion.) 



*> E. M. Kindle : Notes on the Paleozoic faunas of southeastern Alaska. Journal of 

 Geology, vol. xv, 1907, p. 326. 



^1 J. F. Whiteaves : Descriptions of some new or previously unrecorded species of fos- 

 sils from the Devonian rocks of Manitoba. Transactions of the Royal Society of Can- 

 ada, vol. viii, 1890, section iv, p. 93. 



