INTEESPHEEULITIG OEYSTALLIZATIOX. 421 



these lines, appearing as though faulted and displaced to a slight extent. 

 In many instances the arrangement of the streaks of color is such as to 

 indicate that the compact mass was once an aggregate of fragments of 

 similar magma, the lines of flow having various orientations in the different 

 fragments. They have been welded into one continuous homogeneous 

 mass. In those varieties of rhyolite in which the first kind of spherulites 

 are developed — namely, the compact ones which are closely related to 

 microgTaphic intergrowths — there are irregularly shaped areas between 

 bands and clusters of microscopic spherulites which exhibit a different sort 

 of crystallization. They are more highly crystalline — that is, the size of 

 the individual crystals is larger. The spherulites bordering these areas 

 generally terminate in distinct prisms of orthoclase, which have already 

 been described, while isolated crystals of orthoclase of similar habit and 

 size lie in various positions within the more crystalline area. The cement 

 to these crystals in some instances is tridymite, in minute grains or in well- 

 developed twinned crystals with characteristic wedg*e-shaped cross section. 

 In other cases the quartz forms the cement, and is wholly allotriomorphic, 

 producing a micropoikilitic structure (PI. LIV, fig. 4). In places some of 

 the feldspar prisms are nearly parallel to one another, so that the interstitial 

 quartz appears in the shape of thin strips or needles, which is misleading, 

 producing the effect of idiomorphic crystals. This quartz is unquestion- 

 ably of igneous or of aqueo-igneous origin, occupying the same relation 

 to the orthoclase as does the tridymite in the other cases, the development 

 of one or the other depending on very slight differences of physical condi- 

 tion, as demonstrated by experiment. In some experiments both forms 

 have been produced together. Often the tridymite is accompanied by gas 

 cavities of variable dimensions. With the orthoclase and quartz or tridy- 

 mite are some magnetite and small amounts of ferromagnesian minerals. 

 The latter differ in character in different occurrences, and may be fayalite, 

 mica, or tourmaline, and in rare instances possibly garnet. The fayalite 

 forms comparatively large individuals, allotriomorphic with respect to the 

 feldspar, sometimes with an opaque border, which may entirely replace the 

 original individual. 



Tourmaline and mica are found in minute crystals about 0.025 mm. 

 long and 0.01 mm. thick. They are abundant in places, or lie scattered 

 through the tridymite and quartz, and also in the margin of the bordering 



