634 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



the same age, it is probable that the Holiknuk belongs to the Jurassic or the Triassic and that 

 the others are younger. 



Maddren^^® gives the following description, of the Cretaceous in a generalized 

 table of stratigraphic sequence in the central Kuskokwim, Innoko, and lower- 

 central Yukon region: 



Conglomerates, arkoses, and shales, with some coal beds on the Yukon. Contains acidic 

 dikes and sills on lower Kuskokwim and also basic volcanic material. Fragmentary plant 

 remains and marine shells. Probably includes both Upper and Lower Cretaceous. Occurs 

 throughout the lower Kuskokwim Mountains, southeast side of the Innoko Valley, northwest 

 side of lower Yukon Valley, lower Koyukuk Valley, and upper part of central Kuskokwim 

 VaUey. 



O-P 8. SOUTHERN YUKON TEE.RITOBY AND BRITISH COLUMBIA. 



In the vicinity of Lake Bennett (near latitude 60°, longitude 135°) the por- 

 phyrite and tuffaceous rocks of the middle and later Mesozoic are developed with 

 the typical characters of the "Porphyrite group." ' The description by Cairnes"* 

 is in part as follows: 



Newer than these Coast granites is a series [Windy Arm] chiefly consisting of porphyrites, 

 diabases, andesites, tuffs, and basalts which cut through and overlie them, by far the greater 

 part of the rocks being porphyrites which are generally fine-grained and greenish in color and 

 present a fresh, unaltered appearance. 



Following an account of the petrographic character and eruptive relations of 

 the porphyrites, Cairnes continues : 



There have been great showers of ashes and finer material which have consolidated into 

 generally fine-grained greenish rocks showing bedding structure which is often quite pronounced, 

 giving the rocks the appearance of fine-grained sandstones and shales. In places these particles 

 have fallen into water and show to some extent water-sorting action and are mixed with argil- 

 laceous and other sedimentary materials. These rocks seem to grade into true sediments as 

 shales, conglomerates, bedded cherts, cherty quartzites, etc., which carry coal seams. * * * 

 The true sediments with some of the upper bedded ash rocks and tuffs are included in the 

 Tutshi series described below. The rest are included in the Windy Arm series. The whole 

 series appears to correspond to Dr. Dawson's Porphyrite series. * * * This Windy Arm 

 series also corresponds, in all probability, to the rocks of division IV on Mr. Gwillim's Atlin 

 map * * * and includes the corresponding representatives of the greenstones of his gold 

 series division VI, which are newer than the granites. The corresponding representatives of 

 the other members of division VI, which axe older than the granites, have been included in 

 this report, and on the map in the Lower Cache Creek series. * * * 



Tutshi series. — This name was given by Mr. McConnell to include the "argUlites, tufaceous 

 sandstones, conglomerates, etc.," of the Windy Arm district, and the writer has adopted the 

 name for all rocks in the area that appear to belong to this same series. These Tutshi rocks, 

 which in all probability * * * correspond to the rocks of Mr. Gwillim's division III on 

 his Atlin map, belong, as well as the Windy Arm series, to Dr. Dawson's Porphyrite series 

 and correspond lithologicaUy and stratigraphically with his Jackass Mountain series. * * * 

 This latter consists chiefly of igneous products, and the Jackass Mountain series resembles 

 rocks of ordinary aqueous origin. * * * ^yj rocks, therefore, which are of aqueous origin, 

 and all those associated and interbedded with them, are placed in this Tutshi series, as water 

 action has had more or less effect on aU of them and has not affected the rocks of the Windy 

 Arm series. Dr. Dawson considered the Porphyrite series to be probably a link between the 

 Jurassic and Cretaceous, making the Jackass Mountain beds Lower Cretaceous. The fossils 

 found this season in these Tutshi beds, being all typically Lower Cretaceous, further bear out 

 this correlation. 



