THÉ ARCH^AX FORMATION OF THE ABUKÜMA PLATEAU. 235 



appearance. Fine, microscopic, trichitic needles are abundant in 

 the qunrtz, Tlie iir.^t iitduccment to tlu schistositij of our granite is to 

 he found in a slifjlit shifting in the positim of tlie ferro-magncsian 

 silicates, and also in the com men cement nf granulation in the solid quartz. 



In the rock forming- a cheek of the deposit of magnetite in the 

 Chiika valley, whicli may conveniently l)e classed among the [)resent 

 o-roup. I found many patches of the micropegmatite, filhng up the 

 interstitial spaces between the constituent minerals. Felds})ars are 

 beautifully zoned, and polysynthetically twinned ; their peripheral 

 ]>ortion is astonishingly irreguhu', gradually fusing together with 

 the feldspar-substîince of the niicro]}egm:'.tite. The feldspar tbrniing 

 the marginal })orti(ni, above mentioned, has the appearance being- 

 formed l3y the secondary enlargement, repeatedly spoken of, of late, by 

 many American and English authors. It is a subject ot consider- 

 able interest to know v.hether the pegmatite in question is of primary or 

 secondary origin. 



Prof. Jude? recently advocated the view that the formation 

 of pegmatites and grano]jhyres'' may take place after the consolidation 

 of rocks by the way of devitrification, concentration, and differentia- 

 tion of a substance similar to glass-base, if there be any in granites, 

 that is to say, that they are not original but sec(,3ndary products. I 

 found the true pegmatitic structure in some of our schistose granites 

 as, for example, in the specimen from Chûka, a rock changed in its 

 texture bv pressure. This fict seems to have some important bearing 

 on the view of Professor Judd. just stated. Ihit I have also found 

 the same in some normal granites, which have not been even slightly 

 affected by dynamic changes. Under these circumstances no decision can, 



1 Q. J. G. S. Vol. LXV, 1889, p. 179. 



2 Professor Lossen wishes to substitute the word pegmatopliijre for the granophyre or 

 Rosenbusch, loc. cit. p. 270. 



