Fairbanks.] Geology of Point Sal. 89 



the only spot yet known where the feldspathic separation preceded 

 the peridotitic, yet it is certain that the latter is almost invariably 

 followed by dikes of diabasic character. 



The magma basin, from which these numerous bodies of perido- 

 tite were derived, could have been no local one. They occur through 

 nearly the whole length of California west of the Sierra Nevada, 

 and extend for an unknown distance northward. As far as the 

 author's observations have been carried, there appears to have been 

 only one period of intrusion in the Coast Ranges. The eruptions 

 cut strata of Knoxville age and are overlain by the Chico. The 

 diabase sometimes occurs on one side of the peridotite and some- 

 times within it, but at Point Sal, if there can be any distinction 

 made, it is that the magnesian rocks are confined more to the center 

 of the mass, and the feldspathic to the borders. The various masses 

 reached their present position already fully differentiated. Second- 

 ary differentiation took place in both the feldspathic and magnesian 

 facies, but whether the distinct bodies of gabbro, diabase, and peri- 

 dotite are segregations in the common magma basin of the "petro- 

 graphic province" or in the conduit through which the local pro- 

 trusion took place, is difficult to say, although it would seem that 

 the probabilities are in favor of the latter view. 



If the view that the crust of the globe becomes progressively 

 more dense and basic with increased depth is a correct one, then 

 the peridotites when they are not merely facies of more acid rocks 

 must be conceived as originating at great depths. This basic 

 material forced up through fissures over a wide area on the Pacific 

 Coast, occurring everywhere as independent masses, and not mere 

 basic secretions from an acid magma, indicates that it originated in 

 a deeper basic portion of the crust. Under the conditions of great 

 pressure and high temperature existing at great depths, diffusion 

 would be impossible and the composition would remain homoge- 

 neous. The absence of any large bodies of acid rock which appear 

 genetically related to the peridotites strongly supports the view of 

 the existence of a widely distributed basic magma of nearly homo- 

 geneous composition. The differentiation which has taken place in 

 this basic magma in the chimneys has been a very complete one as 

 a general rule. The gabbro is fully as feldspathic near the perido- 



