DEVONIAN 281 



some of the quartz. The latter carry some pyrite. All of the rocks arc 

 characterized by an abundance of pyrite, whose oxidation products have 

 stained them with iron. This pyrite is particularly abundant in the lime- 

 stone, where it occurs in nodules and small seams. 



The whole series is so closely folded that neither the structures or 

 thickness could be determined. It seems probable that its thickness does 

 not exceed 500 feet, but this is uncertain. The relation of these rocks 

 to the adjacent formations was not determined, but they are probably 

 separated from the Carboniferous to the east by a fault, and are overlaid 

 unconformably on the north by the Mesozoic beds. Eocks of a similar 

 character occur on Washington creek, about 15 miles west of the locality 

 described above. The relations shown on the map (figure 2) indicate 

 that the two areas are probably continuous. These rocks were first de- 

 scribed by Collier,'* who correlated them with Spurr's Eampart series. 

 The observations of the writers indicate that they form a belt about 4 or 5 

 miles wide, striking about east and west and bounded both north and 

 south by the younger Mesozoic beds. It should be noted, however, that 

 mapping of the Mesozoic beds to the south is based on lithologic evidence 

 alone. 



This belt of Devonian is made up for the most part of very fine-grained 

 sandstones and quartzites and argillites, with some limestone, together 

 with large masses of greenstone, probably chiefly of a diabasic character. 

 Interbedded with the true sediments are some fine-grained fragmental 

 rocks, having a general greenish color. These, under the microscope, are 

 seen to be made up of fine quartz grains with much calcite and chlorite. 

 Collier called these rocks tuffs, and such they may be, though definite 

 proof appears to be wanting. The greenstones are made up entirely of 

 secondary minerals, in which quartz, calcite, and chlorite dominate. One 

 thin-section revealed what seemed to be the remnants of an ophitic struc- 

 ture, and for this reason, and because of their general resemblance to the 

 greenstones of the same province known to be diabases, these rocks are 

 all assigned to the diabase family. All of these rocks are closely folded; 

 the prevailing strikes are a little north of east, but there is no certainty 

 of dip. They are unconformably overlaid on the north by Mesozoic beds. 



The oldest Devonian rocks of the Yukon which have yielded fossils 

 eml)race a series of blue and white semicrystalline limestones which are 

 exposed at intervals along the river for 15 to 20 miles below Coal creek. 

 These limestone masses are separated by extensive areas of greenstone, 

 Avhich in turn are locallv associated with some cherts and slates. The limc- 



'* Arthur .T. Collier : The coal resources of the Yukon. Bulletin no. 218, U. S. Geo- 

 logical Survey, pp. 28-29. 



