164 



University of California. 



[Vol. 3. 



The method of growth iu the augite seems to be indicated by 

 the following observations: In areas of the cloudy groundmass 

 adjacent to the augite grains, there is seen between crossed 

 nicols an obscure polarization of light. These areas, when the 

 stage is revolved, extinguish as units, generally at the same time 

 with the augite. It appears therefore that most of the augite 

 crystals are surrounded by a sort of delicate sponge of the same 

 mineral, oriented on the parent grain. 



In the third stage of crystallization, the structure might be 

 designated as semi-ophitic. The augite and feldspar, and occa- 

 sionally olivine, may all be distinguished on fresh fractures by 

 the unaided eye, and occasional phenocrysts of feldspar attain a 

 diameter of several millimetres. 



The microscopical structure of a specimen of this type is 

 illustrated in Plate 17, fig. 5. Between large plates of augite, 

 in which abundant feldspar laths lie without any rule of orienta- 

 tion, are inclosed angular or irregular areas of hypocrystalline 

 structure, composed of cloudy glass with crystals of feldspar, 

 olivine, and iron ore. In this case olivine is entirely replaced by 

 iddingsite, which also fills macrolitic cavities into which the 

 feldspar crystals project. Apatite needles are abundant in the 

 groundmass, but rarely penetrate the edges of certain augite or 

 feldspar areas. 



In a fourth specimen, the holocrystalline character is evident 

 in the hand- specimen, where the surfaces of fracture are entirely 

 determined by the rough black crystal grains of augite, with their 

 included strips of bright feldspar. A few large phenocrysts of 

 yellowish feldspar may be seen, as in the type last described. 



Under the microscope, the typical ophitic structure is seen in 

 complete development. The small lath- shaped feldspars and the 

 olivine are idiomorphic against the iron ore, which is in turn 

 idiomorphic against augite. The apatite needles are usually 

 concentrated in certain areas, either of augite or even oftener of 

 feldspar with more or less granular structure, which appear to 

 have been one of the last portions of the rock to crystallize from 

 the groundmass. 



The typical intersertal structure is exemplified in a few 

 specimens. Microscopically, these are seen to be composed of 



