402 II. S. WASHINGTON — IGNEOUS COMPLEX OF MAGNET COVE 



wanting except in the basic, syenitic shonkinite, and biotite is rare ex- 

 cept in the most basic rocks. Magnetite increases regularly as SiO., falls, 

 becoming an important constituent in the jacupirangites. Titanite and 

 apatite are quite common, being abundant in certain types. Orthoclase 

 is the only feldspar observed, the more acid rocks, of course, being the 

 richest in it, though it is found even in some of the basic ones. The 

 total absence of the anorthite molecule is remarkable, in view of the 

 abundance of CaO. It is to be noted that olivine is entirely absent from 

 all these rocks, being found in the region only in a few dikes of monchi- 

 quite, which will be mentioned later. 



GENERAL CHEMICAL FEATURES 



Chemically also these rocks are all related to one another and are all 

 evidently derived from the same magma. They are all essentially soda 

 rocks, this alkali being abundant in all of them and to a large extent 

 giving them their characteristic features. In fact this region is another 

 instance of the well known tendency of soda-rich intermediate magmas 

 to undergo differentiation. Na 2 is constantly greater than K 2 in ratios 

 varying from 1.75 to 4.47, and this ratio increases on the whole with the 

 basicity, as has been found to be the case elsewhere. 



In the next place, they are rich in CaO, the ijolities extremely so, and 

 even 1 the nephelinic syenites carry far more CaO than such rocks from 

 other regions. The rocks are, on the whole, poor in MgO. This, together 

 with the richness in CaO, accounts for the abundance of melanite, since 

 there was not sufficient MgO to combine with all the CaO to form 

 pyroxene. 



Iron oxides offer no especial features of interest, though they are not 

 as abundant in the ijolites as we might expect to find them. No defi- 

 nite relationship between their ratios and SiO., can be made out, but, as 

 has been observed elsewhere in soda-rich rocks,* the ratio of FeO : Fe,0 3 

 is constantly low. Alumina is decidedly high for rocks as basic as the 

 ijolites, and SiO., is, on the whole, low ; in fact, the character of the 

 magma, as a whole, is decidedly basic as well as rich in soda and lime. 



CHEMICAL RELATIONS 



The chemical relations of the various rocks are well seen in the ac- 

 companying diagram, in which total iron oxides are reckoned as FeO. 

 The agreement of A1 2 3 , Na 2 0, and K 2 O with each other, on the one 

 hand, and of CaO, MgO, and FeO, on the other, as well as the antago- 

 nism of these two groups of oxides, are clearly shown. As silica 



♦Washington : Jour, of Geology, vol. vii, 1899, p. 467. 



