1 86 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



H2O below 100° 04 



TiO^ 78 



P,05 Trace 



MnO Trace 



100. 1 1 



Granite. — Associated with the gneiss occurs a rock without band- 

 ing, a normal granite (Cat. No. 75,479). It is probably of similar 

 genetic origin to the gneiss. Its mineralogical constituents are the 

 duplicates of those in the gneiss, even to the merest accessories and 

 alteration products, but the proportions vary, especially that of the 

 micaceous mineral, biotite. Its geological relationships, further 

 than that it constitutes a part of the lower portion of the island, as 

 with the preceding rock, are unknown. It is of medium grain, of 

 pinkish hue, and though a surface sample there are no evidences of 

 alteration visible to the naked eye, with the possible exception of an 

 occasional green speck of chlorite, which may be a product of biotitic 

 alteration. The structure is hypautomorphic granular. 



Under the lens the feldspar is seen to constitute fully seventy-five 

 percent of the rock, the four varieties mentioned with the gneiss 

 being present, viz., orthoclase, albite, microcline, and oligoclase. 

 There is also an excellent development of the micrographic struc- 

 ture. The feldspars have slightly changed, producing kaolin and 

 sericite, and contain many liquid inclusions, as is also the case with 

 the quartz. Strongly pleochroic biotite, slightly altered and fre- 

 quently surrounded by a rim of secondary skeleton magnetite, 

 primary magnetite, zircon with small portions of epidote, chlorite, 

 and apatite, constitute the remaining constituents of the rock. A 

 unique association of original magnetite and zircon was noted as 

 shown indistinctly in plate liv, i. The zircon forms a nearly com- 

 plete ring about the magnetite, its c axis occupying a tangential posi- 

 tion with respect to the periphery of the enclosed mineral. 



Directly back of this low lying expanse, which has been described 

 as stretching a mile back from the coast, there rises to a height of 

 3,700 feet, a typical nunatak, composed of granite (Cat. No. 75,480), 

 essentially similar to that just described. This forms the main 

 residual mass of the granite intrusion, and it is fairly probable that 

 the more basic gneiss already described is simply a differentiated 

 portion of this central magma. Thus it agrees beautifully, so far 

 as position goes, with our usual conceptions of magmatic segre- 

 gation which places the acid phases in the center of an eruptive 

 mass and the more basic phases in the periphery. The difference 

 in elevation of the granite masses is perhaps due to a fault. That 



