156 A. Holmes &, H. F. Harwood — Picrite, Mozambique. 



derived its felspathic constituents by any process of absorption. 

 Had this been the case the radium content would undoubtedly have 

 been higher. 



Cooling History. 



The order of crystallization has already been indicated in the 

 description of the various minerals. It may be summarized as 

 follows : («) magnetite ; (b) olivine, afterwards partly resorbed by 

 the magma ; (c) enstatite ; and (d) augite and calcio-labradorite ' T 

 leaving (e) a residuum of glass. 



This cooling history is of particular interest since it is closely 

 paralleled by that of one of the artificial melts described by Andersen 

 in his paper on the Auorthite-Forsterite-Silica System. 1 The ternary 

 diagram (fig. 9, p. 437, op. cit.) represents the cooling history of 

 any melt containing only the three constituents mentioned. The 

 melt chosen for comparison has a composition equivalent to 60 per 

 cent of forsterite, 15 per cent of silica, and 25 per cent of anorthite. 

 This mixture is expressed in the ternary diagram by a point which 

 lies well within the forsterite field of crystallization. Forsterite ia 

 therefore the first mineral to crystallize, and it continues to do so 

 until the falling temperature brings the system to the boundary-line 

 between the fields of forsterite and clino-enstatite. When this line ia 

 reached 37 per cent of forsterite has crystallized. Now, however,, 

 the further evolution of the system proceeds along the boundary-line 

 in such a way that clino-enstatite is separated, while forsterite is 

 slowly resorbed by the liquid. Ultimately a point is reached between 

 the clino-enstatite and anorthite fields of crystallization, and at this 

 stage 21 per cent of clino-enstatite has crystallized, while the 

 forsterite has been reduced to 34 per cent. During the completion 

 of the crystallization the temperature continues to remain constant. 

 More forsterite is resorbed, reducing its percentage to 25, while 

 clino-enstatite and anorthite crystallize out together, the final 

 proportions of these substances being 50 per cent and 25 per cent 

 respectively. 



In the picrite described in this paper the actual minerals are 

 analogous to those of the artificial melt. Instead of forsterite we 

 have olivine, of which about 80 per ,cent is forsterite ; instead of 

 clino-enstatite we have enstatite, followed by enstatite-augite ; and 

 instead of anorthite we have a calcic-labradorite containing about 

 70 per cent of anorthite. The percentages are also very similar, 

 being approximately in the proportion 30 : 40 : 30, instead of 

 25 : 50 : 25. In an artificial melt of the former composition the 

 ternary diagram shows that the cooling history would be the same as 

 in that outlined above. The crystallization of the picrite was, of 

 course, relatively complicated by the presence of iron and alkalies, 

 but these do not appear to have appreciably affected the salient 

 features of the cooling history. The rock itself, as portrayed by the 



1 Olaf Andersen, Amer. Journ. Sci., xxxix, p. 407, 1915 (see in particular 

 figs. 9 and 11 and table viii). 



