Calkins, I 



Petrography of the John Dai/ Basin. 



135 



are a greenish aug'ite, with an extinction angle near 45° and a 

 fairly pleochroic hypersthene. The latter seems to be slightly 

 more abundant. The slender prisms have" irregular and splintery 

 ends. Prismatic cleavage is visible, and each crystal is broken 

 by numerous transverse cracks. They are characterized l>y 

 abundant glass-inclusions of irregular form. A few crystals are 

 distinguished from the groundmass by greater size, and possibly 

 indicate two periods of crystallization for the mineral. 



The feldspar, which occurs only in the form of microlites, w as 

 determined by the method of Becker, and by extinction angles 

 measured from the a axis on both the braehypinacoid and per- 

 pendicular to it, as acid labradorite, (Abi Aim.). 



The phenocrysts of olivine, though bounded in part by crystal 

 planes, often show evidence of magmatic resorption. While some 

 of the crystals are perfectly fresh, others are almost completely 

 changed over to an olive-green mineral, with the high interference 

 colors characteristic of iddingsite. Here it does not form para- 

 morphs, but rather irregular aggregates. 



The quartz is probably the most interesting constituent. The 

 grains occur, with even distribution, at the rate of about a dozen 

 to a slide of ordinary size. Their outlines never show a sugges- 

 tion of crystal form, but are always the irregular curves produced 

 by magmatic corrosion. Each grain is generally a crystal indi- 

 vidual, but in one case two were observed in contact, separated 

 by an irregular line. Fine irregular cracks are rather numerous. 

 The quartz in one specimen is almost entirely without inclusions, 

 — a single prism of apatite was the only one observed. 



The modifications produced by magmatic resorption are very 

 similar to those described for similar cases by Iddings,* Diller,+ 

 Lacroix,+ and other writers. Each grain of quartz is surrounded 

 by a zone of brown glass, bordered in turn by a wreath of small 

 prisms of pale green augite. The degree of resorption presents 

 all gradations, and there has often resulted the complete des- 

 truction of the quartz, whose place is then marked by a nest of 

 augite prisms. 



•Bull. U. S. G. S., No. 66, 1890, p. 20 et seq. 



tBull. U. S. G. S., Nos. 79, 1891, and 150, 1897, pi. XXXVII. 



X Les Enclaves des Roches Volcaniques, 1893, pp. 17-48, fig. 1. 



