Page:The Journal of geology (IA journalofgeology11893univers).pdf/402

lites, in every respect similar, have been described and figured by Professor Iddings from the Yellowstone Park rhyolites.^[1] While it is not impossible that some of the colorless spherulites are secondary, there is pretty good evidence that many, if not all of them, are primary. These spherulites are embedded in a base which suggests in every way a former glassy condition. In ordinary light there is no appearance of crystallization except the porphyritical. Traversing the groundmass are cracks which occasionally cut directly through a spherulite. Between crossed nicols the field breaks up into a holocrystalline quartz-feldspar mosaic in which the cracks are lost. It seems fair to conclude that the spherulitic crystallization was prior to the cracking, that the granular crystallization is subsequent, and that the cracking took place in an already solidified glass. In these facts we again find obvious indications of a secondary crystallization. In this case the process seems to have been one of devitrification. The other class of spherulites corresponds to those figured by Professor Iddings in Plate XVII.^[2] They are much larger than those which have just been described; the smallest being easily discernible by the unaided eye, and the largest about the size of a butternut. Hence they become a conspicuous feature of the rock as exhibited in the field. They are rarely altogether absent, and in some localities are crowded so close together as to constitute the major part of the rock mass. When without regularity of arrangement, and when brought out in relief by weathering, these spherulites give to the rock a superficial resemblance to a conglomerate composed of rounded pebbles of uniform size and shape. The rich greys, blues, purple and red of the spherulites and matrix render this a conspicuous rock.

Spherulites become an even more striking feature of these rocks when arranged in layers such as have been described in the modern rhyolites of the Yellowstone National Park.^[3] On a face of the rock normal to the layers, they appear as long