204 D. D. CAIRNES SECTION ALONG YUKON-ALASKA BOUNDARY 



logical mapping in that region. Toward the southern end of the section, 

 where a limestone and a shale group are well developed, at one point the 

 limestone group persists upward from Middle Cambrian to Lower De- 

 vonian, and is overlain by the shale group, which there contains Upper 

 Devonian and Carboniferous fossils. Within a distance of 10 miles the 

 same two formations, having changed but slightly lithologically, have 

 a]tered their relations stratigraphically to the extent that there the lime- 

 stone group persists upward from the Middle Cambrian only to the Lower 

 OrdoWcian and is overlain by the shale group, which contains a fauna 

 ranging in age from Upper Ordovician to Carboniferous. 



In addition, a certain amount of light has been thrown on the age of the 

 older schistose rocks of the region. Heretofore these rocks, which have 

 been generally considered to constitute the oldest geological terrane in 

 each district in which they have been studied, were accordingly variously 

 classed as pre-Devonian, pre-Silurian, or pre-Ordovician, according to the 

 age or supposed age of the oldest overlying sediments. It would now 

 appear that this schistose complex of the Upper Yukon basin is at least 

 pre-Middle Cambrian and is in all probability of pre-Cambrian age. This 

 information is of more than ordinary significance, since these rocks are 

 so extensively developed, and since from them has been derived a great 

 portion at least of the placer gold of Yukon and Alaska. 



In conclusion, the writer wishes to state that the work performed along 

 the 141st meridian, as a result of which this paper was prepared, was 

 necessarily very rapidly performed, owing to the inaecessibility of the 

 district. Consequently the information obtained concerning many points 

 is very incomplete, and numerous interesting problems which might other- 

 wise have been solved still remain unsettled ; in fact, the whole work was 

 really that of the pioneer. It is hoped, however, that the knowledge 

 gained will be of some slight assistance in the advance of geological re- 

 search in this Xorthland. 



