884 THE GEOLOGY OF THE NANDEWAR MOUNTAINS, 



SiOo to change olivine to bronzite, and water to break up the 

 original felspar molecules into anorthite and analcite. Zircon 

 (ZrOo) was introduced at the same time. 



Taking all into consideration it appears that there was magmatic 

 mixing^ accompanied by pneumatolytic action. 



N.18. Loc: Sill iii. at Dingo Creek (PI. Hi., fig.l). 



Macroscopic characters: the handspecimen (PI. Iii., fig. la-6) 

 presents a remarkable appearance. It is studded with gigantic 

 phenocrysts and fragments of crystals of a black mineral with a 

 dull lustre not unlike that of gadolinite. In general outline this 

 mineral reminds one of cassiterite, but its good cleavage in three 

 directions and its brittleness show that we have a pyroxenic 

 mineral to deal with. In addition, the rock contains an abun- 

 dance of fragments of crystalline aggregates, many of which have 

 such regular and straight outlines as to be suggestive of pseudo- 

 morphs after olivine. One of the minerals composing the 

 fragments is seen to be a green olivine. Many of the fragments 

 are IJ inches or more in length and over |-inch in width. The 

 black phenocrysts attain a length of 3 to 4 inches, and a 

 diameter af 2^ to 3 inches. In addition we may notice pheno- 

 crysts (up to 1 inch in diameter) of a brown spinel, and a black 

 microcrystalline groundmass. 



The rock weathers to a reddish clay, and decomposing specimens 

 have always a reddish crust of iron oxides. Occasionally large 

 crystals of biotite (or paragonite) attaining a diameter of 1 to 1 J 

 inches, and a thickness perpendicular to the cleavage of about 

 ^-inch, are met with. Calcite or dolomite occurs abundantly, 

 forming amygdules. 



This rock forms a sill, having a thickness of about 3 feet, and 

 dipping S.E. at '2b° (S^ fig. ), capped b}^ and overljang cherty, 

 metamorphosed, Permo-Carboniferous shales containing Glossop- 

 teris, Gangamopteris and Noeggerathiopsis. Intruding the same 

 shales about 30 feet higher up the series we have a sill of felspar 

 porphyry with a trachytic matrix (bostonite). The succession of 



