396 



University of California. 



[Vol. 3. 



crystallization were in progress, the temperature were slightly 

 raised from any cause, the process would be interrupted and the 

 crystallization of the one which was in excess for the new tem- 

 perature would be resumed, till with falling temperature the 

 cryohydrate point were again reached. Under such an assumed 

 fluctuation of temperature near the cryohydrate point for these 

 two silicates, we would have, in the experimental teaching of 

 modern physical chemistry, a consistent explanation of the 

 rhythmical alternation of the crystallization of feldspar alone and 

 feldspar + olivine. 



But while this suggestion may explain the rhythm of crys- 

 tallization or magma differentiation, it by no means explains the 

 specific manifestation of rhythm which is exemplified in the 

 structure of the orbules. Under the assumed conditions of vary- 

 ing temperature in a nearly still magma, the same rhythm might 

 well have found expression in a porphyritic structure or in a 

 laminated structure, or in the form of "basic secretions" or in 

 other ways. We have, therefore, no very clear suggestion from 

 the attempt to apply the phase rule as to the spherically concentric 

 and radial disposition of the products of crystallization, whether 

 at the cryohydrate point or above it . 



The general very fairly uniform size of the orbules and the 

 approximately uniform spacing of their centers in those phases 

 of the rocks which are made up chiefly of orbules are, however, 

 significant. This dimensional factor in the problem seems to 

 indicate the spacial limits of osmotic currents within the time 

 necessary for crystallization. If the distance between centers of 

 the orbules be determined by the rather short distances through 

 which osmotic diffusion operates in limited time, may not the 

 radial structure itself be referred to the radial movement of the 

 diffusion currents? If we make this assumption, then the only 

 question that remains to be answered is why, under the control 

 of these radial currents, the olivine alone assumed the attenuated 

 or rod-like form ; and the answer to this question probably lies 

 in the habit of growth in olivine, dependent primarily upon its 

 molecular structure, but accentuated by favoring osmotic currents. 



University of California, 

 March, 1904. 



