132 G. D. OSBOKNE. 



(A) Albite 29'64% Anorthite 70*56% 



(B) . „ 30-88% ,, 69*12% 



The actual composition of the felspar would be slightly 

 different from that given at (A), in that a little lime must 

 be allotted to pyroxene, with the necessary deduction of 

 small amounts of lime and alumina from the anorthite 

 molecule. However, allowing for such a change the com- 

 position of the felspar as determined optically, corresponds 

 closely to the value obtained from the analysis. 



(d) Harzburgites. — This type of peridotite is by far the 

 most abundant. Its average mineralogical composition, 

 based on rough estimates of the proportions of the minerals 

 in the slides, is approximately as follows: — olivine- 70%, 

 enstatite 18%, diopside 8%, and spinel (picotite) 4%. 



In the following accounts of peridotites the brown spinel 

 will be referred to as picotite. It is not possible to say 

 definitely whether the spinel is picotite or chromite, but 

 the percentage of chromium sesquioxide in two analyses 

 of the Basin rocks is evidently too low to admit of the 

 existence of chromite. 



The harzburgites are coarsely crystalline and variable in 

 grainsize. The component minerals are very much altered. 

 Particular attention has been paid to the significance of 

 the decomposition products of olivine, as regards their 

 relation to the conditions of formation. Thus, as Professor 

 Benson 1 has clearly explained, the alteration of olivine to 

 serpentine and to bowlingite represents a dual set of con- 

 ditions, the production of serpentine being effected at great 

 depths, and the changes to bowlingite and iddingsite being 

 wrought by katamorphic agencies. 



In the harzburgites complete pseudomorphs of olivine by 

 bowlingite do occur as well as mesh-serpentine replace- 



1 "Origin of Serpentine," Am. Journ. Sci., December 1918, p. 693. 



