A. Holmes & H. F. Harivood — Picrite, Mozambique. 153 



analysis indicates that the composition of the olivine includes nearly 

 80 per cent of forsterite, the specific gravity should he about 3*4. 

 This figure would give for the rock a calculated specific gravity of 

 3 1 10, which agrees more closely with the observed result. 



Chemical Composition". 



An analysis of the rock was made by Dr. H. F. Harwood, with the 

 following results : — 



Total 



100-16 



Specific gravity (average of three specimens) = 3-08. 



Eadium content (A. H. 1915) = 0-44 x 10~ 12 grams per gram of rock. 



The composition of the rock is of a type not very commonly found. 

 Its nearest analogue is that of a Hawaiian picrite-basaltf which was 

 erupted in 1840. 1 A few other similar analyses are cited in the 

 table below (p. 154), from which it may be seen that two British rocks, 

 one from Anglesey and one from Loch Garabal, have a general 

 chemical similarity to the picrite under discussion. The presence of 

 nickel and chromium in every case in which it has been sought for is 

 an interesting featui'e, and indicates the importance of making 

 analyses as complete as possible. The association of these elements 

 with olivine-rieh rocks has been frequently pointed out, 2 and there is 

 no doubt that by determining such relations in detail much may yet 

 be learned concerning the genesis of igneous rocks and ore-deposits. 

 Among African rocks the Mozambique picrite resembles most closely 

 some of the mineralized picrites of the Insizwa Range. 3 Owing to 

 the presence of pyrrhotite and of copper and nickel sulphides in these 

 rocks the analyses cannot be directly compared, but it is clear that 



1 W. Cross, Prof. Paper 88, U.S.G.S., 1916, pp. 44, 77. The mineralogical 

 composition of the Puna lava of 1840 is almost identical with that of the 

 Ampwihi picrite. 



a H. S. Washington, Trans. Ann. Inst. Min. Eng., xxxix, p. 735, 1908. 



3 W. H. Goodchild, "Economic Geology of the Insizwa Eange " : Inst. 

 Min. and Met. (read December 21, 1916). 



