Fairbanks.] Geology of Point Sal. 2$ 



of the rock is exceedingly variable, as is also that in different parts of 

 the same hand specimen. It does not seem to bear any definite 

 relation to different portions of the crystals, being in some instances 

 more pronounced in the rims, in others at the center or along the 

 line marking the transition between the basic interior and the acid 

 exterior. The change to analcite, as in the Cuyamas dike, is very 

 widespread, but other cloudy decomposition products are abundant. 

 In one specimen the alteration appears as a dark almost opaque 

 band running concentrically around the crystals at the junction of 

 the acidic and basic portions. The feldspars inclosed in the large 

 poikilitic crystals are as a rule very fresh, while those aggregated 

 together between the augites, having associated with them the 

 greater part of the analcite, are often reduced, together with the 

 analcite, to an indeterminable cloudy mass. The feldspars are quite 

 fresh in some of the darker rocks which appear much decomposed 

 through the leaching out of the analcite. As a rule the alteration 

 of the feldspar is bound up in the closest manner with the presence 

 of the analcite. The feldspars within the augite, comparatively free 

 from contact with the analcite, are fresh ; while without, where the 

 most of the analcite occurs, the alteration is often extreme. 



Twinning occurs on the albite law, though ; n some cases it is 

 combined with the carlsbad law. The twins consist of broad and 

 narrow plates. The feldspars in the poikilitic augite crystals are in 

 the form of small laths, sometimes not averaging more than a milli- 

 meter in length. They occur either singly or in aggregates. With 

 an increase of the feldspars in the augites there is a transition to the 

 ophitic structure, where the feldspars are distributed nearly evenly 

 through the rock. In one section a linear arrangement of the feld- 

 spars is very noticeable. 



An interesting micro-pegmatitic intergrowth is present in one of 

 the coarser dark facies (Plate 2, Fig. 2). This is probably second- 

 ary and is taking place on the borders of the broad feldspar plates. 

 It is an intergrowth of two feldspar individuals differently orientated. 

 In some cases it has to a considerable extent replaced the original 

 feldspar. The outer portion of this intergrowth is made up of large 

 areas, irregular in outline, but becoming smaller towards the heart 

 of the original crystal, and terminating along crystallographic 



