GENERAL GEOLOGY 35 



SiO, 57.62 



Fe,0 s , Al.0.1 Mn,0 4 24.47 



CaO 8.74 



MgO 1.34 



K,0 .30 



Na 2 O i.oo 



Loss on ignition (H 2 O) 8.58 



1 02. 05 l 



The basalt of the dikes is a very fine-grained dull black rock, con- 

 taining veins of chalcedony and calcite. It has an exceedingly fine 

 ophitic structure, with much magnetite. 



The other specimens were found as beach pebbles. 



Apltte (164) . A yellowish-white compact rock, much finer grained 

 than the aplite from Plover Bay (described later) but much like one 

 from St. Lawrence Island. It shows, over the whole slide, an ex- 

 ceedingly beautiful fine-grained micropegmatitic groundmass surround- 

 ing small feldspars with raveled-outends. There are occasional larger 

 phenocrysts of oligoclase, some quartz grains, and shreds of biotite. 



Pyroxene -tonalite (i6j). A fresh and wholly granitoid rock of 

 rather coarse, even grain, and with a pearl grey color dependent on the 

 fact that the feldspars which make up most of the rock are blackish 

 translucent in the central half and opaque flesh-colored on the outside. 

 It contains much pyrite and little of any dark constituent. The feld- 

 spars are largely idiomorphic and strongly zoned. They are near albite- 

 oligoclase, and are completely kaolinized inward to a sharp boundary. 

 They are surrounded by a very complete micropegmatitic structure, 

 which radiates out from them, growing coarser out ward, and the lobes 

 of the feldspar are seen as well in common light as with polarized 

 because of the kaolinizing. The dark constituent is a pale pyroxene, 

 nearly all changed to hornblende. 



Augite-porphyrite ( i6j) . There is a brownish-red base with small 

 brick-red porphyritic feldspars. The microscope shows it to be quite 

 fresh, but with fissures and cleavage planes filled with red rust. The 

 feldspars occur in very broad twinning bands, with extinction 38-39 

 on the albite twinning plane, and so are very basic labradorite. The 

 ground is hyalopilitic with a remnant of devitrified glass. 



1 Fe was weighed as Fe a O 3 when some of it possibly was FeO. Mn was 

 weighed as Mn 3 O 4 when it undoubtedly existed as MnO. This brings the total 

 above 100 per cent. 



