220 C. N. Fenner — Crystallization of a Basaltic Magma 



PetrograpMc Description of the Watchitng Basalt. 



In those portions of the Watchung flow in which chilling* 

 was most rapid the surfaces of the pahoehoe-like bosses were 

 crusted with much glassy material. Apparently, however, 

 judging from the study of a great number of slides, crystalli- 

 zation was under way and phenocrysts had begun to appear in 

 the mass of flowing lava before the sudden chill occurred which 

 stiffened it into a glass. The resultant glass shows a typical 

 vitrophyric texture. The well-developed phenocrysts are sur- 

 rounded by isotropic glass or by a ground mass in which glassy 

 material is more or less mixed with the feathery forms of micro- 

 lites. Under the generally accepted hypothesis, in a typical 

 diabase consisting of magnetite, plagioclase, and diopside, the 

 magnetite would be first to form, and its elimination from the 

 magma would be complete before the plagioclase began to 

 appear. These two are held to be followed by the third constit- 

 uent, diopside, which occupies the space left by the other two. 

 On the other hand, the manner in which the minerals should 

 crystallize out, as demanded by the principles of eutectiferous 

 solutions, is quite different, and in every case the sections exam- 

 ined conform to the latter requirements. No matter to what 

 degree glass may be present in the slide, plagioclase and 

 diopside appear side by side. It is evident that almost from 

 the beginning of crystallization these two constituents were 

 being eliminated simultaneously. At times it appears that the 

 diopside is in somewhat preponderant amount, and at other 

 times the plagioclase. 



With regard to the magnetite the evidence is more obscure. 

 The proportion of magnetite in the magma is not large and it 

 seems to appear first as a dark dust. The exact point at 

 which elimination from the fusion began is not clear, but it is 

 certain that growth of magnetite grains continued while 

 diopside and plagioclase were still forming. 



Slide No. 105 shows a typical development of glass and 

 phenocrysts. (See fig. 1.) 



Probably three-fourths of the section is isotropic glass, 

 uniformly pale green or light olive-green in the thin section. 

 Scattered throughout are numerous phenocrysts of plagioclase 

 and diopside. The plagioclase is in typical lath-like crystals 

 reaching an ordinary maximum of 0'2 — 0"3 mm long by *05 mm 

 wide. The diopside is developed in more nearly equidimen- 

 sional forms of a characteristically stouter appearance. An 

 exceptionally large crystal measured 0*6 by # 2 ram . The out- 

 line is sometimes octagonal, but in general irregular. In 

 many cases the plagioclase and diopside are closely associated, 

 small clusters of mutually intergrown individuals lying in 

 groups surrounded by glass. It is noticeable in such cases 



