494 THE MAEQUETTE lEON liEAKING DISTRICT. 



chemical composition is iu the neighborhood of this mineral, is embedded 

 in the mass and so gives a porphyritic habit to the sections. Leucoxeue is 

 abundant. The original structure of the rock is obscured by the abundant 

 secondary substances occumng in it, Ijut the ophitic texture is clearly 

 appai'ent. In one phase (Specimen 22169) the amphibole Is more fibrous 

 than in the others, but in the center of an amphibole cluster a small core 

 of augite was noticed. 



The rocks nearer the periphery of the mass (Specimens 22166, 22170, 

 and 22172) show glistening areas of hornblende and small brilliant laths 

 of plagioclase in a dull-green groundmass. In thin section they are appar- 

 ently porphyritic, for large crystals of feldspar are embedded among the 

 small laths of this mineral and the amphibole which together make up the 

 larger portion of the rocks. None of these rocks differ essentially from 

 those above mentioned. Chlorite is more abundant in them, but the 

 amphibole is of the same light-green color, and is present in the same 

 ophitic areas. The plagioclase is more altered than in any of the other 

 rocks mentioned, but its original outlines can still be recognized. 



At the very edge of the knob is a dai'k-green schistose phase of the 

 rock. The hand specimen is made up of small, dark, glistening areas 

 resembling those of amphibole. The thin section is a nearly uniform aggre- 

 gate of tiny chlorite and small brown biotite flakes, grains of magnetite 

 or ilmenite, and very small grains, sometimes laths, of clear plagioclase. 

 This luiiformity is broken in places by lenses of altered plagioclase, in 

 which badly defined feldspathic substance is cut by spicules of chlorite. 



In the study of these sections from a single rock mass we become 

 acquainted with the different forms which may be assumed by a mediumly 

 coarse grained diabase under the influence of processes that are mainly 

 weathering and metasomatic phenomena but partially phenomena due to 

 dynamic agencies. The original rock was a coarse diabase. This has given 

 rise to epidiorites containing secondary fibrous hornblende, to "diorites" in 

 which the amphibole is a compact and sometimes an idiomorphic variety, 

 and to uralite-diabases. Where mashing has taken place iu addition to the 

 weathering, chlorite-schist has been produced. 



There is not a particle of evidence in these sections that the schist or 

 any of the forms of the massive greenstone were ever fragmeutal rocks. 



