8 1 8 THE JO URNAL OF GEOLOGY. 



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 consti- 

 tute 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 



x Opus cit, PI. XVII., p. 276. 



2 Opus cit. p. 277. 



3 Iddings: opus cit. p. 276, PL XVIII. 



