National Geographic : 1898 Apr
ALASKA AND ITS MINERAL RESOURCES rocks, called the Fortymile 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 blendic, 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 source of gold in the placers. The younger rock series noted are, briefly, the following: lThkandit series.-This consists of limestones, sometimes white and crystalline, generally green or black, alternating with shales.