1'ETKOLOGICAL NOTES ON FUUTHKU K< >CK SPECl.MKNS. 321 



III. ADDITIONAL PETROGRAPHIC NOTES ON ROCKS FROM 



MADIGAN NUNATAK. 



1. INTRODUCTION. 



A brief summary of StillwelTs description of the locality and the kinds of rocks 

 found on the Nunatak is necessary. 



He says (p. 128), " the Madigan Nunatak is situated in Lat. 67 8j' and Long. 

 143 20', about 30 miles distant from Cape Denison . . . , and 18J miles from Cape 

 Gray." 



He gives photographic views of the Nunatak (Plate XXIV, figs. 1 and 2). 



He further states, "it is composed of gneissic rocks whose foliation strikes 

 approximately north and south " ; and " two rock types are found in this area. One 

 is a black massive plagioclase-pyroxene-gneiss " which " seemed to form a band whose 

 trmd cuts at right angles across the foliation. The second type is the n. ore abundant 

 acid gneiss, containing blue quartz and hypersthene. In the neighbourhood of the 

 ant it -line it has a banded character, but in other parts the gneissic character, though 

 evident is less prominent." 



2. THE PLAGIOCLASE-PYROXENE-GNEISS. 



(a) Modal Composition of Rocks. 



Stillwell has taken specimen No. 794 as the standard of this type of rock. Four 

 other specimens were collected by Stillwell's sledging party in the summer, 1912-13. 

 He has not given a description of any of these specimens, viz., Nos. 775, 783, 788, 792. 

 They are readily seen to be akin to the standard but differences are n< ne-t he-less readily 

 observed. 



In the hand specimens the rocks are black, dense and fine-grained. They 

 resemble very fine-grained dolerites when viewed macroscopically. The minerals seen 

 in the hand specimens are pyroxene and felspar. As in the case of No. 794 the weathered 

 surfaces of these specimens are discoloured by a brown iron stain. 



The textures of the rocks shown by the microscope are granoblastic. "with 

 subsequent modification by cataclastic effects." The average grain-size is about 0-25 

 nun., although No. 783 is somewhat finer, its average grain-size being approximately 

 0-17 mm. 



:) B 



