BY H. I. JENSEN. 607 



forming phenocrysts which may include olivine. The phenocrysts 

 are quite allotriomorphic, and often bound together in such a 

 way as to indicate that the rock is derived from the refusion of 

 a coarsely crystalline gabbro or theralite. The second generation 

 occurs in minute idiomorphic grains in the base, and sometimes 

 ophitically intergrown with felspar. 



Darker brown titaniferous augite and greenish varieties allied 

 to segirine occur in some of the rocks. 



' (e) Apatite is a common constituent in minute quantities, and 

 occurs in the form of long needles often included in the felspar 

 and augite phenocrysts. 



(f ) Magnetite in idiomorphic cubes and ilmenite in hexagonal 

 plates are both very common. 



(g) Sodalite or analcite with very low refractive index and 

 completely isotropic is a common constituent. It occurs in 

 perfectly clear but very irregular patches in the interstices 

 between the other minerals. It stains strongly. 



As inclusions, in the alWite phenocrysts particularly, we find 

 olivine, apatite, magnetite, augite, segirite, and occasionally 

 biotite. 



An interesting point is that in these rocks the felspar pheno- 

 crysts are always corroded less than those of augite and olivine. 

 It appears therefore that the basic nature of these rocks is due 

 to the remelting and absorption of a gabbro, theralite or essexite 

 by an acid alkaline magma. An acid magma would exercise 

 greater corrosive chemical) influence on the basic minerals of 

 the absorbed rock than on the acid ones. The felspars would 

 only be slightly corroded, and would rather tend to grow as the 

 magma cooled. We should therefore expect, and actually do 

 find, zoning common in the felspars of these basic rocks. 



W.67. Loc: Tonduron (The Spire), head of Spire Creek. 

 (Plate xxxi., figs. 5-6). 



1. Handspecimen a dark bluish-black rock with splintery 

 fracture and oily lustre. 

 47 



