282 BROOKS AND KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



stones probably form tbe base of this succession, as will be shown below. 

 Its fossils are correlated with those from a limestone of Middle Devo- 

 nian age on the Porcupine. The igneous rocks and associated slates and 

 cherts are undoubtedh' Devonian and in general younger than the lime- 

 stones. They may belong either to the Middle Devonian or represent the 

 UjDper Devonian shale series foimd near Eagle and to be described below. 

 It should be noted that heavy cherty limestone and associated rock':., 

 which are traversed by the Yukon a few miles below "Woodchopper creek 

 and have been already descril)ed and provisionally assigned to the lowest 

 Devonian or Silurian, occur in this same general area. 



The limestones of this series occur in small patches, often representing 

 small anticlines and apparently inclosed within the greenstone. It is 

 essentially a blue cherty limestone, but locally has been altered to a semi- 

 crystalline phase, apparently due to the metamorphic effect of igneous 

 rocks. On the lower part of Woodchopper creek and at a few other local- 

 ities black argillites are associated with the limestones. The limestone 

 was found only in the northern part of the area within a few miles above 

 and below the mouth of Woodchopper creek. In the northern two-thirds 

 of the area only igneous rocks were observed, with the exception of some 

 small belts of red and green siliceous slates and cherts which occur within 

 the greenstones. 



The igneous rocks which make up this complex have been fully de- 

 scribed by Spurr,'° and it is not intended to go into the matter here. 

 They appear to be in a large measure effusive rocks, but intrusives are 

 not uncommon. In general, both the extrusive and intrusive phases are 

 of basaltic or diabasic character, but they vary greatly in composition. 

 Bedded tufaceous rocks are not uncommon, and some of these appear to 

 grade into imjDure sediments. 



In the hand specimens the rocks are characterized throughout by a 

 green color. They are usually jointed and sometimes locally schistose. 

 At several localities some coarse volcanic breccias were observed. The 

 anticlinal structure of some of the limestone masses occurring in this 

 complex has already been referred to, and beyond this there is little to be 

 said of the altitude of the sediments. The igneous material is usually 

 quite massive, though there is an occasional suggestion of different lava 

 flows among the effusives and bedding among the tufaceous rocks, and 

 marked jointing was noted throughout the greenstone series, producing 

 blocky angular fragments. 



The igneous series has a marked effect on the topography: where it 



'» J. E. Spun- ; The geology of the Yukon gold district, Alaska. Eighteenth Annual 

 Report, U. S. Geological Survey, part iii, pp. 161-166. 



