THE EASTEEN AEEA. 417 



it is determined to be probably not so basic as labradorite. Its alteration 

 ])roducts are kaolin, sniarag-dite, chlorite, andepidote. Augite, the last min- 

 eral to crystallize, maintains relations with plagioclase and magnetite snch 

 as are usnal in diabases. It has generally altered to a larger extent than 

 the feldspar, the resultant products being usually biotite and chlorite, 

 although smaragdite is found. 



The exposures in the northeast })art of Sec. 24 are not greatly dif- 

 ferent from the above, except that they contain no augite, and therefore 

 nuist be classed as porphyrites rather than augite-porphyrites. In all 

 probability they were originally nearly alike. These rocks arc liolo- 

 crystalline. In some cases the background is composed very largelv of 

 small, somewhat altered jdagioclases, but in other cases the smaragdite is as 

 plentiful as the feldspar. Aside from these two minerals secondary (piartz, 

 chlorite, epidote, and gray, nonpolarizing material aie fciund. The minor 

 minerals are somewhat confusedly mingled with the 2>i"edominant ones. 

 The constituents completely interlock, and there is no doul)t that the rock is 

 a crystalline eruptive. The 'exposures in the north part of Sec. lo resem- 

 ble closely the last described, except that they are more finely crystalline, 

 the backgrounds of the liner grained ones being very minutely crystalline. 

 The additional differences are, that the plagioclases lie with tlieir longer axes 

 in a common direction (which probably indicates a tlowage structure), that 

 menaccanite is abundant in small crystals and minute grains, and that its 

 alteration has resulted in leucoxene, which ap[)ears to be of the nature of 

 titanite. The secondary minerals are the same as in the previously described 

 exposures. 



The exposures in Sec. 14 contain rocks in which the groundmass is 

 of agray a morphous or feebly polarizing material, which is taken to be 

 devitrified glass, and also holocrystalline kinds. The glassy backgrounds 

 include minute, much altered tabular plagioclases and particles of material 

 which may be leucoxene. Less abundantly are found other miuerals which 

 have been mentioned as occurring;' in the j)reviously descril)ed exposures. 

 From this glassy variety the background varies to one which resembles 

 closely the exposures in the north part of vSec. 15, except that alteration has 

 extended farther. In some cases the feldspar has very largely decomposed 



MON XIX '27 



