190 



University of California , 



[Vol. 3. 



From a comparison of the nine plots, numbered in accord- 

 ance with Table I, it is seen, first, that the normal diorites are 

 more acid than the two main rocks of these notes. This is 

 shown by the plots of the diorites being farthest to the right, 

 their shorter Al-Fe line, Ca-Mg line, and little longer Na-K line. 

 Second, the rocks I and II show undoubted similarity to the 

 hornblende-gabbros, the Al-Fe line only being a trifle shorter, 

 and to the diorite IV of Rosenbusch, a rock rich in hornblende. 

 Thirdly, the gabbro (gabbro-diorite of Williams), analysis X, 

 shows more basic than all, although poorer in iron. Therefore, 

 from these plots, it appears that from the point of view of com- 

 position of igneous magmas, the Pajai-o rocks are nearer the 

 gabbros than the diorites. 



CONCLUSION. 



The igneous rocks at Pajaro represent a good example of the 

 differentiation of igneous magmas. The oldest phase, the fine 

 grained diorite, is the most basic — an ultra-basic diorite. The 

 next in point of age is the main mass of the rock, also a basic 

 diorite. In this phase, the change from the crystallization of 

 the basic minerals to the acid was sudden, as is attested by the 

 sharply zoned feldspars and little interstitial quartz. Through 

 these two rocks were later intruded acid dykes of pegmatite and 

 aplite, high in silica and the alkalies, often containing 50 per 

 cent, orthoclase. 



The dioritic rocks had best be called basic diorites, the great 

 balance of their characteristics falling on that side. Their actual 

 nomenclature has, however, disclosed the lack of conformity in 

 rock classification, and it is to be hoped that these notes may be 

 of some small service in calling more careful attention to the 

 general question of the nomenclature of the more basic igneous 

 plutonic rocks. 



University of California , 

 May, V.i 02. 



