THE BASIC MASSIVE ROCKS, ETC. 709 



instances sends out tongue-like processes that penetrate far into 

 the plagioclase in which the pyroxene is imbedded (see Fig. 5), 

 so that there can be no doubt that the conditions were favorable 

 to the formation of intergrowths between these two minerals 

 during the period when they were separating from the rock 

 magma. The only essential differences between the fibrous 



Fig. 5. Diallage plate and olivine grain in plagioclase. The augite in the bend 

 extends out into the feldspar, giving rise to an intergrowth, very like that of the 

 fibrous rim. 8803. X ca. 20. 



intergrowths and that illustrated in this figure are, first, the finer 

 structure of the former, and second, its occurrence around the older 

 components of the rock. Neither of these differences is impor- 

 tant, however. Only the second needs a moment's consideration. 

 The position of the fibrous growth around the olivine and 

 other minerals is due not necessarily to the fondness of the inter- 

 growth for this place, but simply to the fact that the diallage, 

 during the earlier stages of its growth, fastened itself to the solid 

 particles in its vicinity and coated them with an envelope of its 

 material. Continuing its growth it formed the encircling rims of 

 this material that are so characteristic of many specimens of the 

 gabbro, and, when the feldspar began to separate it formed with 

 this the granophyric intergrowth. Since the position of the dial- 

 lage had already become fixed, the intergrowth naturally was 

 compelled to occupy a place just without this and around the 

 minerals which the diallage had already partially or entirely 

 encircled. 1 Though a fibrous intergrowth of pyroxene and plagio- 

 clase with the aspect of a reaction rim surrounding the older min- 

 erals of a rock is a rare phenomenon, it is not a unique one, for 



1 For fuller description of the intergrowth, see author's paper in Am. Jour. Sci. 

 XLIIL, 1892, p. 515. 



