ALASKA AND ITS MINERAL RESOURCES 157 



rocks, called the Forty mile series, because of their development 

 on Fortymile creek. They are characterized by alternations of 

 beds of marble, from a few inches up to 50 feet in thickness, 

 with quartzitic and other schists, which may be micaceous, horn- 

 blenclic, or garnetiferous, and sometimes graphitic. They are 

 traversed by abundant dikes of eruptive rock, mostly granites 

 and diorites. Two sets of quartz veins are developed in these 

 rocks: (1) an older set, which are generally parallel to the 

 schistosity or lamination, like those in the Birch creek series, 

 and like them are broken by later movements and carry pyrite 

 and occasionally galena ; (2) a set of larger veins, which form an 

 apparent transition from dikes of aplite, a rock consisting of 

 quartz and feldspar. They cut across the bedding and are not 

 disturbed by later rock movements, hence are younger in age. 

 Rampart series. — This still later series is primarily distin- 

 guished from the preceding by the darker color of its rocks, 

 which are dark green when fresh and become a dark red by 

 weathering. They consist largely of basic eruptive materials, 

 beds of diabase and tuffaceous sediments, with hard green shales 

 and some limestones containing glauconite, or green silicate of 

 iron. They also contain novaculites, or fine-grained quartzitic 

 slates, and jasperoids, or iron-stained quartzose rocks. Serpen- 

 tine and chlorite, noticeable by their softness and green color, 

 are frequent alteration products. These rocks also contain a 

 few quartz and calcite veins, which are generally developed 

 along shear zones, or places where by rock movement and com- 

 pression a series of closely appressed parallel fractures are devel- 

 oped. The basic character of these rocks and their large content 

 of pyrite seem favorable to the concentration of ore deposits; 

 they present, moreover, certain analogies, both in composition 

 and in geologic position, with the copper-bearing rocks of Lake 

 Superior. But the observed veins are younger than the joints 

 and shear planes, which were probably produced by the rock 

 movements that crushed the veins of the older series, and assays 

 of their ores have as yet shown but insignificant amounts of gold 

 and silver. These veins, as well as those in the granite, are, 

 moreover, much less abundant than those in the Birch creek 

 and Fortymile series ; hence it is thought that the latter are 

 probably the principal sour.ce of gold in the placers. 



The younger rock series noted are, briefly, the following: 

 Tahkandit series.— This consists of limestones, sometimes white 

 and crystalline, generally green or black, alternating with shales. 



