Fairbanks.] 



Geology of Point Sal. 



79 



in the peridotite. The two are not always distinct, but the division 

 will aid in understanding the phenomenon. The first is conceived 

 to be due to movement during or just preceding the crystallization 

 in a homogeneous magma; the second, to the slight mixing during 

 movement of a heterogeneous magma. A description of the former 

 will be taken up first. 



In some portions of the gabbro which are perfectly massive 

 there is noticeable a slight tendency to the segregation of the differ- 

 ent components. In this part of the rock, as well as in that which 

 is homogeneous and massive, there could not have been much 

 movement during solidification. Suppose, however, that in such a 

 magma, in which there is a tendency to local segregation, a 

 differential movement should take place, there would result a draw- 

 ing out of the segregated parts of some or all of the constituents to 

 form a gneissic structure. This is best illustrated in the case of the 

 troctolite where the olivine is drawn out in irregular elongated 

 patches which, when they are discontinuous, simulate very closely 

 the structure of a gneiss, without, however, any tendency to schis- 

 tosity. Wherever these shreds become continuous, the different 

 characters may sometimes be observed in the same hand specimen, 

 and there is presented a rude banding. From the latter to the most 

 perfect banding there are all degrees of transition. The feldspars 

 between the shreds or bands of olivine show no common orientation 

 or other signs of having undergone movement during crystalliza- 

 tion, and it must be held that the banding was assumed before the 

 crystallization of the feldspar at least. The olivine may have crystal- 

 lized before or during the movement, probably the latter, and to this 

 fact be due its occurrence in aggregates of many small grains. 



In the case of the troctolite there is no possible support for the 

 view that the gneissic structure is due to the mixing of heteroge- 

 neous magmas. Given certain conditions of the magma favorable 

 to the segregation of the different components, perhaps in this case 

 of the olivine alone, and a movement of the mass on itself, and the 

 structure described could conceivably be produced. This structure 

 is in no way due to the linear arrangement of the individuals of 

 either of the components, for they have a typical granular form, 

 but rather to the lineation of aggregates. From the almost massive 



