RHYOLITK. 375 



(]Uiirty. and t'chlspar, with occasidiially a little Itiotite. The iiiii-rosciii»k-al habit of these 

 porphyritical crystals is so constant in all the thin seetions of this };T<iup as to permit 

 of a single detailed description, the dittereut modifications of the grouTidmiiss only 

 requiring sjjecial notice. The feldspar present is sanidine, with which plagioclase is 

 associated to a greater or less extent. The latter is in some cases entirely wanting, 

 hut in others is almost as abundant as the sanidine. Sometimes both occur in very 

 small quantities in the thin sections and hardly ever outnumber the quartz. Sani- 

 dine occurs m well developed crystals and also in angular fragments. Sections of 

 the former are mostly rectangular, with the corners rounded: otheis show more than 

 four sides and indicate that their < rystal lorm is made up of OP, odPoo, ocP, 2Pao 

 Zcmal strncture is rarely observed. .Many of the individuals are in Garl.sbad twins. 

 The cleavage is frequently \ery perfect, tiu)ugh often entirely wanting, but there 

 are always concoidal fractures, and the resend)lance to ijuartz is often very striking, 

 rccpiiring an optical test to distinguish between them. It is cluiracterized by a nuich 

 lower double lefraction, which in these extremely thin .sections causes it to remain 

 generally dark or but faintly lighted between crossed nicols. (^uite a number <if 

 sections happen to be nearly at right angles to the optical bisectrix and «'xhibit very 

 small angles between the optic axes, the interference figure being almost a cross and 

 showing the bisectrix negative. There are several of these in thin section 112. A 

 fortunate section i)arallel to the clinopiuacoid occurs in thin section 142 and is at 

 right angles to the optical normal, which is found to be po.sitive, the interference figure 

 being hyq)erbolas that unite in the center of the field. The inclinatioTi of the plane of 

 the optic axes is about + 7° to the basal cleavage, and the iiiigles of the sides of the 

 feldsi)ar .section (correspond to those cut from OP, xP, and 2P i/-. liesules the basal 

 cleavage, which in this section is very i)eifect, is a second less regular cleavage 

 parallel to the trace of the orthopinacoid. Tiie i)laiic of the ojitic a.xes in these sani- 

 dines is sometimes in the i)hiue of symmetry, sometimes at right angles to it. The 

 substance of the sanidine is very |)ure and free from inclusions of foreign matter. 

 Numerous minute gas cavities, however, occur irregularly scattered, some of which have 

 their sides wet with fluid, but the gas has always the greater volume. A notable 

 excei)tion to this freedom from inclusions occurs in thin section 141, Fig. 2, PI. v, 

 where two sharply outlined crystals of sanidine grown together with dittereut orien- 

 tation about a fragment of plagioclase arc (illccl with (juart/, in orderly arranged 

 forms, with constant crvstallogra])hic orientation throughout certain portions of the 

 feldspar crystals, which is shown by the extinction (tf light and the parallel ])osition of 

 numerous small dihexahedral glass inclusions with gas bubbles, found ouly in the 

 (luartz, whilst irregularly shaped gas cavities oc^cur in the feldspar substance. This 

 is a most interesting I'act from its relation to the subject of tluid and glass inclusions 

 in volcanic rocks, for it would ajipear from this instance thatcpiaitz aiidfcldsitar crys- 

 talli/ing out at the same time aiul iiiidcr the same conditions have inclosed, the one 



