ACID VOLCAI^^ICS OF HEMLOCK FORMATION. 85 



the structure wlierethe quartz. is surrounded by this micropoikilitic zone, he 

 calls it the granophyric structure. As I have already emphasized above, 

 the feldspars in the network of quartz have varying orientation, and the 

 structui-e is, strictly speaking, microj^oikilitic, and in no sense granophyric 

 (micropegmatitic). Moreover, he describes in addition to the above type 

 one in which are found phenocrysts of quartz Ipng in a micropoikilitic 

 groundmass with the above reticulating texture, but the phenocrysts abut 

 sharply against the groundmass, instead of being connected with it by 

 means of these zones. 



The mici'opoikilitic texture has been held in some cases to be of sec- 

 ondary origin and the result of devitrification. While recognizing that 

 there may be certain unquestionable cases where a micropoikilitic structure 

 results from the devitrification of a glassy groundmass, I can find no evi- 

 dence in the rocks here described that points to this origin for the micro- 

 poikilitic texture under discussion. On the other hand, there is an absence 

 of evidence that indicates its unquestionably primary character. Rather 

 than to regard the quartz as secondary and influenced in its orientation by 

 the phenocrysts, as in the enlargements of quartz grains, it seems natural 

 to suppose that when the lava was extruded after the crystallization of the 

 phenocrysts, there began, consequent upon the diminished j)ressure and 

 temperature and other factors, a rapid crystallization of the mineral elements 

 from the remaining magma. This resulted in the production of the feldspar 

 in very imperfect and small crystal individuals. At the same time the quartz 

 of the phenocrysts continued to grow, and in so doing inclosed these small 

 feldspars in its meshes. 



In certain rhyolite-porphyries the micropoikilitic texture is somewhat 

 different from that above described. In these the quartz phenocrysts are 

 surrounded by zones which are illustrated in figs. A and B, PI. XXI. 

 These appear to correspond very closely to the ones described by Michel 

 L^vy^ and Williams,^ and since described by many other writers. The 

 zones have a much higher index of refraction than the quartz of the 

 phenocrysts, and hence contrast strongly with it. Examined closely, 

 they are seen to be composed of chlorite, epidote, and black or reddish 



'Annales des Mines, A'ol. VIII, 1875, pp. 378, 381. 



-Die Eruptivgesteiue der Gegend von Triberg im Schwarzwald, by G. H. Williams: N. Jahrb. 

 fur Min., Bd. II, 1883, p. 605. 



