OUTLINE OK GEOLOGIC HISTORY 313 



sea was of wide extent, but the fragmental rocks of the Eampart region 

 indicate that there was a landmass not far away. It is probable, though 

 not proven, that the intrusion of the granites now preserved in part as 

 augen-gneisses took place in Siknian or early Devonian times. The con- 

 ditions during the earliest Devonian time, like those of the preceding 

 periods, are very much in doubt. The heavy limestone and associated 

 rocks of the Upper Yukoir section, here assigned to the early Devonian, 

 indicate deep-sea conditions; but, again, these may be of Silurian age. 

 It seems probable tliat one of the recurrent epochs of crustal disturbances 

 took place in early Devonian or late Silurian times, for all the rocks laid 

 down Ijefore this period appear to fall in the metamorphic class, though 

 they have suffered a varying degree of alteration. It is probable that 

 this epoch was closed by an uplift followed by erosion. 



In the Yukon section proper the oldest definite stratigraphic tie-point 

 is a white crystalline limestone cari'ying a Middle Devonian fauna, and 

 this horizon appears to be widespread in Alaska, indicating that much 

 of the province was submerged during this period. On the Yukon this 

 was accompanied and probably succeeded by extensive extravasation of 

 igneous rocks. Volcanism continued until thoiisands of feet of material 

 had accumulated. There is some evidence that this period of igneous 

 activity, though intense, was very local, and that during it normal sedi- 

 mentation was going on in near-by localities. 



In any event, deposition continued unbroken from the latter part of 

 the Devonian into the Carboniferous, the only change being a gradual 

 lessening of land-derived material and the appearance of a new fauna. 

 Deep-sea sedimentation continued for a long period, and in a sea which 

 probably covered most of Alaska. A second C3^cle was inaugurated by an 

 uplift accompanied by extensive erosion and the deposition of coarse 

 clastic material, in part, probably, laid down in fresh water. The period 

 of erosion was long enough in some parts of the province to remove all 

 tlie strata down as far as the metamorphic terrains. 



The close of the Carboniferous was marked by a widespread invasion 

 of a sea which probably covered all Alaska and much of the adjacent 

 provinces. In the Copper Kiver region late Carboniferous sedimentation 

 was accompanied by another volcanic outburst. There is some evidence 

 to suggest that in the Yukon l:)asin there was no break in sedimentation 

 between the Carboniferous and the Triassic. Jurassic rocks are unknown 

 on the Yukon, and as the Lower Cretaceous rests imconformably on the 

 older terrains, it is probable that tliey were preceded by a cycle of erosion. 

 The folding of the Lower Cretaceous beds marked the close of extensive 



