Reid.] 



The Igneous Bocks near Pajaro. 



1ST 



NOMENCLATURE. 



General Discussion . — In regard to the nomenclature of these 

 rocks the question of their exact classification naturally arises. 

 Unfortunately, it is yet impossible to assign the igneous rocks to 

 their proper place on but one basis; there is still a clash between 

 the purely mineralogical basis of classification and the purely 

 chemical. 



Chemically, the two principal rocks discussed in these notes 

 may be either diorites or gabbros; and II is much nearer a 

 typical gabbro in composition. In Table I is given an average 

 analysis of diorites by Pirsson, column VI, and another by 

 Brogger, in column VII. That of Brogger comes nearest repre- 

 senting a typical diorite. In comparison, it is seen that in \ II 

 the silica is considerably higher, while the iron, lime, and mag- 

 nesia, are lower, than in I and II, II being decidedly more basic. 

 The soda is approximately the same, but the potash is higher 

 in VII, due to some orthoclase being present. Most of the 

 potash belongs with the ferro-magnesian minerals, however. 

 Columns VIII and IX give two auylyses of hornblende-gabbros. 

 Of these, the silica is lower, and the lime and magnesia higher, 

 than in the diorites already cited, VI and VII. The alumina is 

 also higher in the hornblende-gabbros, and the alkalies are about 

 the same. These two rocks are, therefore, decidedly more basic 

 than the diorites. Column X, the gabbro-diorite of Williams, 

 may be taken as a gabroitie rock. On comparing it with II, the 

 more basic of the Pajaro rocks, the main differences are the 

 higher silica, alumina, lime and magnesia, and the lower iron 

 and alkalies; II, therefore, is a little more acid than X. Com- 

 paring II with VIII and IX again, it may be seen that II has a 

 lower alumina content, yet on the whole is more basic. As 

 regards I, it is more acid than these gabroitie rocks cited, but 

 corresponds well with analysis IV, a hornblende rich diorite cited 

 by Rosenbuseh. Both I and IV are more basic than the typical 

 diorites. Chemically, therefore, I stands intermediate between 

 the diorites and gabbros, while II takes its place with the 

 gabbros. 



Mineralogically, the Pajaro rocks, I and II, are unequivocally 

 diorites, being composed of hornblende and a medium basic 



