280 BROOKS AND KINDLE — PALEOZOIC ROCKS OE UPPER YUKON 



shales and slates, with some quartzites and a little tuff. All these rocks 

 are closely folded and sometimes faulted, and there are probably duplica- 

 tions of the same beds. Therefore, though these rocks outcrop for several 

 miles, they may not exceed a few thousand feet in thickness. The ab- 

 sence of greenstones, with the exceptions of a very subordinate amount of 

 tuff, differentiates these rocks from the other Devonian terrains of the 

 adjacent regions. In general the succession can be said to be calcareous 

 and argillaceous with a striking absence of igneous and fragmental ma- 

 terial. This succession is sufficiently distinctive to warrant giving it a 

 formation name, .were it not that its limits are too ill defined and its 

 stratigraphic position too uncertain. In considering the age of the rocks, 

 it should be noted that the oldest Devonian of the Porcupine is a lime- 

 stone carrying Middle Devonian fossils, and that this rests on Silurian 

 beds. If the series above described is Devonian, it represents a formation 

 which is entirely lacking in the Porcupine section. 



Some 2 or 3 miles below the mouth of Fourth of July creek a belt of 

 limestones, cherts, with some interbedded carbonaceous clay shales and 

 slates, together with considerable limestone conglomerates, are exposed 

 on the west bank of the Yukon, which are here provisionally assigned to 

 the Devonian without paleontologic evidence. The stratigraphic position 

 of these rocks, like that of the series described above, is very uncertain. 

 The lithologic character of these sediments would suggest that they be- 

 long with the cherty limestones, etcetera, already described and assigned 

 to the oldest Devonian or Silurian, but their intimate association with 

 igneous rocks points to a correlation with the limestone-greenstone series 

 of Middle Devonian age to be described below. 



The dominating rock of this belt is a blue cherty limestone which, adja- 

 cent to some intrusives, becomes quite crystalline. In places the limestone 

 is interbanded with chert layers; in others, with beds of carbonaceous 

 shales. Some beds of limestone conglomerate or breccias were found 

 interlarded with the other sediments. In these the fragments are in part 

 well rounded, suggesting water action; in part, quite angular, and may 

 be frictional breccias. The cement of some of these conglomerates, which 

 is iron stained, was believed in the field to be tufaceous, but a microscopic 

 study of the thin-section shows it to be purely of sedimentary origin. 

 Greenstone intrusives, chiefly of diabasic or basaltic character, are very 

 extensively developed in association with these rocks. They occur chiefly 

 as dikes, possibly also as sills. That the shearing which produced the 

 limestone breccias was subsequent to the intrusives is shown by the fact 

 that many of the dikes themselves are badly fractured and deformed. In 

 places there are many calcite veins between the igneous rocks as well as 



