Augite and Plagioclase in a Minnesota Gabbro. 519 



For evidence as to the nature of the constituents of the 

 intergrowth an appeal must be made to the fibrous growth 

 itself. But this is not as satisfactory as it might be, for the 

 fibers in most cases are so fine that nothing more definite can 

 be detected in the rim than an intergrowth of two distinct 

 substances. In instances where its fibrous structure is less well 

 developed, and its components are coarser, the two minerals 

 composing the rim are seen to differ in their colors between 

 crossed nicols, as already indicated. One of these is in long 

 narrow stringers, with club-like extremities. It appears to be 

 continuous with the plagioclase on one side of it and to extend 

 between similar tongues of a more highly refractive substance 

 that spring from the opposite side. It is occasionally marked 

 by twinning bars that are continuations of those existing in 

 the surrounding plagioclase, and it extinguishes simultaneously 

 with the latter. It is undoubtedly plagioclase. 



The more highly refractive mineral intergrown with the 

 feldspar is occasionally in such coarse fibers that its augitic 

 character cannot be cpuestioned. In the section of rock No. 

 8800,* for instance, a large grain of olivine is penetrated by 

 two lath-shaped crystals of plagioclase. Between the feldspars 

 and the olivine is a very narrow rim of augite, while perhaps 

 half of the material within the outlines of the crystals consists 

 of the fibrous intergrowth. On the side of the intergrowth 

 toward the augitic rim, its highly refractive component assumes 

 such broad dimensions that diallagic characteristics may be 

 traced in it, far beyond the points where it leaves the rim to 

 form prolongation's between the pla- 

 gioclase fibers. Fig. 5 illustrates a case 

 in which augite sends out into the pla- 

 gioclase in which it is imbedded, long 

 pseudopodia-like tongues. Many other 

 instances like the last might be cited 

 to show the tendency of the diallage 

 to form intergrowths with the plagio- 

 clase, but enough has already been 

 written to emphasize the point. 



It is true that there is no absolute Fig. 5. Augite plate and 

 proof that the highly refractive sub- ° l i vine g^in in plagioclase. 



r , „ , „ ]?, J ,, The augite in the bend extends 



stance of the fine fibrous intergrowths ou t into the feldspar, giving 



is identical with the material of the rise to an intergrowth, very 



coarse prolongations just mentioned, ]l ^® that ° f A tbe fibrous nm - 



but the series of gradations between 



the mantles surrounding olivine and the fibrous intergrowth is 



*A11 numbers of rocks refer to the series belonging to the Lake Superior 

 Division of the U. S. Geological Survey. 



Am. Jour. Sci— Third Series, Vol. XLIII, No. 258.— June, 1892. 

 34 



