1 .AWSON. I 



Malignitc. 



as an aggregate of small grains in between the augites. The neph- 

 eline may be discriminated from the feldspar as dull white spots, 

 which are quite abundant, but which present no sharply recogniza- 

 ble boundaries to the unaided eye. The apatite is remarkably 

 abundant and occurs in elongated grains ranging up to 3 mm. in 

 greatest diameter, which by reason of their yellow color, vitreous 

 lustre and rather high refractive power, are prominent features of 

 the hand specimens. The pyroxene prisms are several times 

 longer than their breadth. The planes in the prismatic zone are 

 well developed, but no terminal planes could with certainty be 

 detected. The prisms have all possible orientations. The biotite 

 occurs in plates which occasionally attain a diameter of 5 mm. It 

 js, however, usually much smaller, and is a subordinate constituent 

 of the rock. 



The general gray color of the rock is occasionally blotched with 

 darker patches, in which the pyroxene, in very much smaller indi- 

 viduals, preponderates greatly over the light-colored constituents. 



Structure. — Under the microscope a remarkable feature of the 

 rock is the poikilitic relation of the orthoclase to all the other con- 

 stituents. Over large areas it extinguishes uniformly between 

 crossed nicols, thus proving its physical continuity, although in the 

 plane of the section it frequently appears in discrete areas. In 

 several sections about 2x1.5 cm - m extent only one individual of 

 orthoclase is present, and in this are imbedded all the other con- 

 stituents. It is evident from this relationship that the orthoclase 

 was the last mineral to crystallize, and that it represents, in fact, the 

 residual magma after the other constituents had separated. None 

 of the other minerals thus inclosed in the orthoclase appears to 

 have any definite relation in space with reference to the host, but 

 have a perfectly haphazard orientation. The large crystals of 

 orthoclase, which thus serve as the paste of the rock, have no 

 geometrical boundaries but are allotriomorphic with reference to 

 one another. The fundamental structure of the rock may, there- 

 fore, be said to be allotriomorphic granular, and all of the con- 

 stituents other than the orthoclase are of the nature of idiomorphic 

 phenocrysts. None of the minerals of the rock seems to have 

 separated in more than one generation if we leave out of considera- 



