A Summary of Progress in Petrography. in 1887-1896

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. Excerpt from book: Section 1MINERALOGY AND PETROGRAPHY.1 Petrographical News.?The eruptive rocks of Velay, Haute Loire, France, in the order of their age are basalts, trachytes and tra- chytic phonolites, augite andcsitcs, porphytic basalts, nepheline pho- nolites and nepheline basalts. Termier, '' who describes them, gives but a few brief notes on each type. The younger phonolites form the larger part of the hill. They contain aegerine in light-green porphyritic erystals, and in microlites. At the south-east of St. Pierre-Eynac are tertiary clay slates cut by dykes of phonolite, whose tiny veins penetrate metamorphosed phases of the elastics, and are thus consequently regarded as the agents producing the alteration. The rocks representing the first stage in the alteration consist of granitic debris, in which secondary opal has been deposited around the feldspar and quartz fragments. In some instances, in addition to the opal there have been formed also secondary quartz and calcite, the former as a fibrous rim around the grains. In more intensely changed phases, the slate is traversed by veins of phonolite, whose contact with the sedimentary rock is not visible, since on both sides of it the material of the phono- lite has thoroughly impregnated the slate. On the other hand the phonolite of the veins contains sphene, but no augite, while the normal rock contains an abundance of aegerine, but no sphene. In the final stage all the quartz of the slate has disappeared, and the rock is comprised principally of opal, serpentine and clay (halloysite?), with pleo- naste, colorless augite and hornblende as new products. The alteration is thus a silicification. In other, more rare cases, it is a feldspathiza- tion.?Hutchings, has recently studied the material of which slates are formed, having examined for this purpose, clays and micaceous sa..