ACID VOLOAi^ICS OF HEMLOCK FORMATION. 83 



erals were present must have beeu scattered tlii-ough the groundmass, and 

 have been completely altered. The secondary minerals contained in the 

 groundmass are chlorite, calcite, epidote, muscovite, and biotite. 



TEXTURE OF THE PORPHYRIES. 



The texture of the dense groundmass varies according to the mode of 

 association of the two chief minerals — quartz and feldspar. The commonest 

 variety is the rhyolite-porphyry with microgranitic groimdmass (poiphyre 

 granulitique of Michel Ldvy). A second variety is the rhyolite-porphyry 

 with micropoikilitic groundmass.^ The microgranitic texture is too well 

 known to warrant a description of it here. 



The micropoikilitic texture presents certain characters which render a 

 ftu'ther description desirable. This peculiar phase of the micropoikilitic 

 textiu-e was briefly described by the writer and illustrated by microphoto- 

 graphs in 1895.^ Shortly after the separates of this article were distributed, 

 I received from H. Hedstrom, of the Swedish Greological Survey, an article 

 published in 1894 containing a description of what appears to be very 

 nearly the same texture.^ If I have understood the description correctly, 

 however, there seems to be one essential diiference. In order to explain 

 clearly this difference, I shall describe the textm-e in detail. 



In certain of the rhyolite-porphp-ies, as already mentioned, the quartz 

 phenocrysts are surrounded by certain zones. These zones in the rocks 

 having a micropoikilitic texture possess exactly the same texture as does 

 the groundmass. The zones are composed of minerals wliich are of suffi- 

 cient size to permit readily their determination. Quartz and feldspar are 

 the essential components, with some clilorite, epidote, muscovite, and iron 

 oxide. The first two are the important minerals, and will alone be referred 

 to in the further description. The chief peculiarity of the zone is in the 

 arrangement of the two minerals, and this character is best shown on the 

 accompanying microphotographs. This textui-e can be seen even in 

 ordinary light. It is brought out better when the field is partly shaded, so 



' Eruptive rocks of Electric Peak and Sepulchre Mountain, by J. P. Iddings : Twelfth Ann. Kept. 

 U. S. Geol. Survey, 1891, p. 589. On the use of the terms poikilitic and micropoikilitic in petrography, 

 by G. H. Williams : Jour. Geol., Vol. 1, 1892, pp. 176-179. 



2 Volcanics of the Michigamme district of Michigan, by J. Morgan Clements: Jour. Geol., Vol. 

 in, 1895, pp. 814-816, figs. 1 and 2. 



^Studier ofver Bergarter fran Moran vid Visby, by H. Hedstrom: Geol. Foren i Stockholm 

 Forhandl, Bd. 16, H. 4, 1894, pp. 5-9. 



