Fairbanks.] 



Geology of Point Sal. 



85 



a small scale in certain portions of the magma as the temperature 

 lowered prior to consolidation, and in conjunction with this a move- 

 ment of the magma on itself took place in the conduit. These two 

 things, taken together with the fact that there were slight differences 

 in the time of crystallization of the different constituents, would 

 tend to produce linear aggregations of more or less purity. It 

 seems that the greater part of the segregation took place during 

 the movement, but that this was not a segregation of partially con- 

 solidated material, except perhaps the olivine, for the crystals of the 

 different bands present no more appearance of movement during 

 their period of formation than do those of the massive varieties. 

 The layers interlock as Geikie and Teall have described. It is quite 

 probable that the olivine, which always occurs as aggregates of 

 numerous small grains, crystallized during the movement, but in 

 the case of the feldspars movement must have ceased when they 

 began to form. None of the other dark silicates show such 

 peculiar stringy masses as the olivine. 



Discussion of Magmatic Variation. — It is not the plan here to 

 enter into any detailed discussion of the question of differentiation in 

 rock magmas. The work done in this line has been thoroughly 

 discussed by Iddings, BrOgger, Rosenbusch, and others. The se- 

 quence, however, of the intrusions at Point Sal will be discussed, as 

 well as their relation to similar rocks in California and other parts 

 of the world. 



Iddings * states the general law of differentiation to be as fol- 

 lows : "The variation in the composition of igneous rocks, which 

 constitute a series of eruptions at any volcanic center, is the result 

 of the chemical differentiation of some intermediate magma." "The 

 general succession is from a rock of average composition through 

 less silicious and more silicious ones to rocks extremely low in sil- 

 ica and others extremely high in silica, that is, the series commences 

 with a mean and ends with the extremes." 



Geikie f has formulated the general succession of eruptive rocks 

 in Great Britain as follows : " With the important exception of the 

 Snowdonian region, and possibly others, we find that the earlier 



*Bull. Phil. Soc. Wash., 1892., pp. 151, 145. 

 t Quart. Jour, of the Geol. Soc, Vol. XLVII. 



