THE POEPHYRITES AND ANBESITES 



81 



olivine. Apatite and the iron ores are almost universally pres- 

 ent, and often picotite, ehromite, pyrrhotite, more rarely com- 

 mon pyrites, and a green spinel. Secondary brown mica and 

 hornblende are common. Quartz occurs but rarely. 



Chemical Composition. — As with other groups, the percent- 

 age amounts of the various constituents obtained by analyses 

 is dependent upon the relative proportion of the constituent 

 minerals. In the tables given below, analyses like I and III, 

 showing very little iron and magnesia, but rich in lime and 

 soda and alumina, are of rocks in which the pyroxenic con- 

 stituents are almost wholly lacking, and which consist essen- 

 tially of lime feldspars only. 



Chemical Composition of Gabbro 



Constituents 



I 



II 



III 



IT 



V 



YI 



Silica (SiOa) .... 



59.55 % 



64.72 % 



58.43% 



49.15% 



46.85 % 



45.66 % 



Alumina (AleOg) . . . 



25.62 



17.79 



28.01 



21.90 



19.72 



16.44 



Ferric iron (Fe20s) . . 



0.76 



2.08 



0.75 



6.60 



3.22 



0.66 



Ferrous iron (FeO) . . 



* . • • 



6 03 



.... 



4.54 



7.99 



13.90 



Lime (CaO) 



7.73 



6.84 



11.24 



8.22 



13.10 



7.23 



Magnesia (MgO) . . . 



Trace 



6.85 



0.63 



3 03 



7.75 



11.57 



Potash (K2O) .... 



0.96 



3.01 



0.96 



1.61 



0.09 



0.41 



Soda (Na20) 



5.09 



3.02 



4.85 



3.83 



1.56 



2.13 



Ignition and loss . . . 



0.45 



.... 



... * 



1.92 



0.56 



0.07 





100.15^ 



99.34% 



99.83% 



99.80% 



100.84% 



98.07% 



I. Anortliosite : Chateau Eicher, Canada (T. S. Hunt). II. Gabbro: near 

 Cornell Dam, Croton Eiver, New York (J. F. Kemp). III. Anorthosite: 

 Labrador (A. Wickman). IV. G-abbro: near Buluth, Minnesota (Streng). 

 V. Gabbro: near Baltimore, Maryland (G. H. Williams). YI. Gabbro: 

 Northwest Minnesota (W. S. Baylej). 



Structure. — The gabbro structure is quite variable. Like 

 the other plutonic rocks mentioned, they are crystalline granu- 

 lar, the essential constituents rarely showing perfect crystal 

 outlines. As a rule the pyroxenic constituent occurs in broad 

 and very irregularly outlined plates, filling the interstices of 

 the feldspars, which are themselves in short and stout forms 

 quite at variance with the elongated, lath-shaped forms seen in 

 diabases. This rule is, however, in some cases reversed, and 

 tj 



