42 



conditions, consolidate as a crystalline mass almost entirely composed of 

 augite ; but if it be maintained at a very nigh temperature during the initial 

 stages of consolidation and then allowed to cool somewhat more rapidly both 

 olivine and augite will be formed. (1) In this case we see that the development 

 of olivine is dependent on the physical conditions of consolidation. The 

 researches of Messrs. FOUQUE and LEVY (2) on the artificial production of 

 rocks and minerals may also be referred to for illustrations of the effect of 

 physical conditions in determining the mineralogical characters of rocks. 



One of the most interesting constituents of certain volcanic rocks is the 

 glassy or crypto- crystalline matter in which the crystalline constituents are 

 embedded. In many cases this may certainly be regarded as representing 

 the mother-liquor remaining after the separation of the crystalline constituents 

 from a magma having the same composition as the entire rock A 

 comparison, therefore, of the analysis of this interstitial matter with the 

 bulk-analysis of the rock will throw light on the changes which occur in the 

 liquid magma in consequence of the successive formation of crystals. It is 

 unfortunate that we do not possess at present a large number of analyses 

 available for the purpose of instituting such a comparison ; nevertheless, those 

 which we have are sufficient to suggest general conclusions of considerable 

 interest. 



I. IA. II. IlA. 



. 66-25 



18-59 

 3-11 



2-75 

 28 

 495 

 2-25 

 5-89 



98-56 100-00 98-51 99-08 



Sp. Gr. ... 2-77 2-543 2-437 



(1) BECKEE, A. Schmeltzversuche mit Pyroxenen und Amphibolen. Z.D.G.G. Band 

 XXXVII. 1885, p. 10. 



(2) Synthese des mineraux et des roches. Paris, 1882. 



