6 N. L. Bowen — Genetic Features of 



a section the minerals monticellite and melilite may be 

 fonnd without even incipient change. The monticellite 

 is apparently considerably more susceptible than melilite. 



Variation in the Principal Type. 



The greater part of the mass appears to be essentially 

 uniform in hand specimens and in order to gain an idea 

 of such variability as might exist it was necessary to 

 resort to collection of specimens regularly distributed 

 over the outcrop. Some thirty specimens were collected 

 and they show considerable variation in the proportions 

 of the minerals. Usually biotite is the most abundant 

 mineral and is always an important mineral, but it is not 

 infrequently exceeded in amount by monticellite and 

 sometimes by melilite or chrysolite. Melilite may be 

 present to the extent of about 30 per cent and again may 

 occur in vanishingly small quantity. Augite may amount 

 to about 20 per cent but is usually less and there is a 

 general tendency for monticellite to occur in greater 

 quantity where augite is more abundant. Chrysolite 

 may rise to 35 per cent or again sink to 10 per cent. 

 Pyroxene, monticellite and melilite were practically 

 absent in only one specimen, giving essentially an ordi- 

 nary mica peridotite (see Plate I (c) ). 



Chemical Composition of the Principal Type. 



In view of the variability of the rock it was desirable 

 to choose for analysis a specimen which appeared under 

 the microscope to represent the general average. At the 

 same time it was desirable to choose material which was 

 as unaltered as possible, and this latter restriction made 

 it necessary to take material differing somewhat from the 

 average. The actual specimen chosen contains the 

 constituent minerals in approximately the following 

 proportions in weight per cent: chrysolite 30, melilite 

 25, biotite 20, monticellite 10, augite 6, all others 9. No 

 great accuracy is claimed for these figures because the 

 rock as a whole is rather unfavorable for the estimation 

 of proportions of the minerals. The general average of 

 the rock would be decidedly higher in monticellite, con- 

 siderably lower in melilite and chrysolite and somewhat 

 higher in biotite and augite. 



