324 E. M. KINDLE RECONNAISSANCE OF PORCUPINE VALLEY 



of either impossible. The thickness of the older series exposed, however, 

 is probably not far from 600 feet. 



In the Yukon section the Ordovician does not appear, the oldest beds 

 being' of late Silurian age. If we may judge from the widely scattered 

 localities from which it has been reported, the Ordovician fauna had a 

 wide distribution in the northern part of the continent. 



Collier's collections from the Seward peninsula indicate the presence 

 in the western part of Alaska of a fauna similar to that found on the 

 Porcupine. Madurina manitol ensis is one of the species recorded by 

 Collier.^ 



In the Mount McKinley region Brooks has secured fossils of Ordo- 

 vician age.^ Ordovician faunas are therefore known from the three 

 widely separated regions of northeastern, south-central, and western 

 Alaska. 



In the Arctic and sub-Arctic lands of northeastern America Ordovician 

 strata have been shown by the collections secured by Schuchert,* Schei,*^ 

 Low," and others to have a rather wide distribution. Of these the collec- 

 tion from Baffin Land studied by Schuchert is the only one which is 

 complete enough to give an adequate idea of the Ordovician of the north- 

 eastern portion of the continent. 



SILURIAN DOLOMITES 



The low, nearly continuous cliffs which border the Porcupine river for 

 about 20 miles just above the northeastern margin of the Yukon flats are 

 called the Lower Eamparts. A Tertiary basin which the Porcupine 

 crosses separates these lower cliff walls of the river from another and 

 much higher series of canyon-like walls known as the Upper Eamparts, 

 which extend from the boundary, with some interruption, down to the 

 Coleen basin. 



Buff-colored magnesian limestones comprise the greater part of the 

 rocks of the Lower Eamparts. Only a rough estimate of the thickness 

 of these limestones can be made. Probably 3,500 feet of these dolomites 

 are exposed in the Lower Eamparts. Some black shaly beds are asso- 

 ciated with them, but comprise only a small proportion of the series. 

 Occasional drab magnesian shales occur and rarely a thin bed of quartzite. 



2 Professional paper no. 2, U. S. Geological Survey, 1902, p. 21. 



^Bulletin of the U. S. Geological Survey, 1904. (In preparation.) 



* On the Lower Silurian (Trenton) fauna of BaiBn Land. Proceedings of the U. S. 

 National Museum, vol. 22, 1900, pp. 143-177, pis. 12-14. 



^ New Land : Four years in the Arctic regions, hy Captain Otto Sverdrup, with geo- 

 logical appendix to vol. 2 hy P. Schei, 1904, pp. 457-458. 



oThe cruise of the Neptvne. Report on the Dominion government expedition to Hud- 

 son bay and the Arctic islands, hy A. P. Low, Ottawa, 1906, pp. 322-336. 



