192 r>. D. CAIKXES SEX^TIOX AIX)NG YUKON-ALASKA BOUITDAKY 



been deposited largely along the bedding planes of the containing rocks 

 in seams ranging from microscopic up to 8 or 10 inches ia thickness, and 

 thus gives the rocks in general a decidedly banded appearance. When 

 somewhat regularly deposited along the bedding planes in this way, the 

 chert has in places the appearance of being contemporaneous with tiie 

 containing beds, but when more closely examined it may be seen to inter- 

 sect the strata ; in fact, seams or masses of chert occur cutting the lime- 

 stone and dolomite beds at all angles, and the smaller seams are frequently 

 distinctly traceable back to larger seams or irregular bodies. 



The entire series of rocks rangiug through the Silurian, OrdoTician, 

 and Cambrian appears to be conformable throughout, and it was found to 

 be impracticable, if not impossible, to differenriate and map separately the 

 beds of the different ages. The rocks are all so much folded and faulted 

 that only in a few plac-es could the positions of the different beds within 

 the series be even approximately determined stratigraphic-ally : and unless 

 fossils could be found it was impossible, even in these places, to draw the 

 geological age-boundaries, as no distinctive persistent lithological horizon 

 markers could be distinguished, such as occur in less uniform forma- 

 tions in many districts and serve to indicate the position of geological 

 boundaries in the absence of fossil remains. FobslIb, moreover, are of 

 somewhat rare occurrence in these beds, particularly in the lower mem- 

 bers, so that a great amount of detailed paleontological and stratigraph- 

 ical work would be required to subdivide these rocks and map them ac- 

 cording to their respective ages. 



The Devonian beds, however, are dominantly or entirely limestones, 

 which have an aggregate thickness of from 300 to 500 feet^ In places it 

 is difficult or impossible, except paleontologicallv, to distinguish these 

 rocks from the underlving limestone beds, but as a rule they are some- 

 what more homogeneous and darker in appearanc-e, being typically dark 

 bluish gray in color. They are also in most plac-es more c-oarsely ciystal- 

 line. and when broken they characteristically emit a strong oily odor, 

 which was not noted to the same extent in connection with the underlving 

 formations. In places a heavy bed or series of beds of white to light gray, 

 sugar-grained quartzite oc-curs at the base of this series, but this appears 

 to be only locaUy developed. These beds, wherever a contact was observed, 

 overlie the Silurian beds unconf ormably. 



The Devonian limestone corresponds somewhat closely to the Salmon- 

 trout limestone on Porcupine Eiver, as described by Kindle,^ and the two 

 formations may be the same. It would seem probable, however, that the 



» E. M. Kindle : BnlL GeoL Soc Am.. toL 19. Oct., 1908. pp. 327-329. 



