THE BASIC MASSIVE ROCKS, ETC. 



701 



There can thus be no doubt that the feldspar throughout the 

 entire mass of the rock is practically of the same character, since 

 the three specimens tested were taken from three widely separ- 

 ated portions of the gabbro area, and each represents a distinct 

 type of the rock. No. 8786 is very rich in olivine, No. 8589 

 contains much augite and a large quantity of brown biotite, while 

 No. 10440 is very rich in feldspar and quite poor in pyroxene. 



An analysis of the feldspar separated from No. 8786, and 

 partial analyses of the plagioclase from the other rocks were made 

 by Dr. W. H. Hillebrand. They are as follows: 



The figures under 8786 and 8589 correspond very closely with 

 those of a basic labradorite. Those under 10440a and 10440b 

 are abnormal, in that they indicate that the more basic portion 

 of the feldspar in this rock has a lower specific gravity than the 

 more acid one. The alumina in the four cases, however, corre- 

 sponds quite well with the proportion of this oxide in basic 

 labradorites. In Ab 1 An 3 , which Tschermak makes the dividing 

 line between labradorite and bytownite, the percentage of alumina 

 present is 32.8 per cent. Since the rock specimens from which 

 these feldspars were separated represent the only phases of the 

 gabbro that have retained the normal gabbro characteristics, it is 

 probable that the feldspars themselves represent the variations 

 within whose limits all of the feldspar in the great mass of the 

 rock may be found. A comparison of this plagioclase with that 

 of the very coarse diabase from the boss-like dike forming Pigeon 

 *A11 iron determined as Fe 9 0,. 



