44 MDDDLEJOSS: THE GEOLOGI OF IDAR STATE. 



Wttb this end in view, it is obviously necessary to turn to one 



factor in fche problem that manifestly might 



Suggested contact seein m ore 01 less possibly accountable for 



effect of the aplite ^ spec j a Uy mineralised condition of the calc- 



gneiss, namely, the plexus of aplite veins, or 

 possibly the intrusive masses of 'the Idar granite. Neglecting the 

 letter for the moment, does the calc-gneiss throughout its great 

 thickness and great development of calc-silicate minerals, owe 

 its present condition to thermal and contact effects of the aplite 

 veins on lime-bearing sediments? Here the great doubt that 

 obtrudes itself is the small volume of the vein material compared 

 with that of the rock intruded by it. Notwithstanding the wide 

 dissemination of these veins and feheii striking, white appearance, 

 the total mass of them appearing at the surface is not very con- 

 siderable. At p. 33 T have defined a characteristic section across 

 half a mile of calc-gneiss country. There at most, 12 veins of 5 

 feet thickness each can be allowed. That is to say a thickness of 

 60 feet of the aplite must be accountable for thermal contact effects 

 extending through an apparent thickness of more than 2.500 feet; 

 or, distributing the 12 veins equally across the section, the gaps 

 would be 220 feet in width across which the contact action of 5 

 feet of igneous rock would have to be imagined as operative. 



These conditions certainly seem to imply an insufficiency 

 of igneous rock to account altogether or primarily for the highly 

 mineralised condition of the gneiss, though at the same time we 

 cannot disregard the circumstance of their ever-present ramifica- 

 tion through the calc-gneiss in a very intricate way, and also the 

 possibility that they may connote the presence of more extensive 

 parent granites situated possibly not far below the present surface. 



Turning now to the masses of the Idar granite, it may be re- 

 marked that the addition of idocrase to the 



Suggested influence ca lc-gneiss, developed in considerable quantity 

 of the Idar granite. ^.^ ^^ tQ ^ ^^ ftnd nowhere 



else (see p. 20) seems to mark one narrow aureole of this 

 contact influence, a conclusion borne out by a very similar 

 development of idocrase quite close (within a few feet of) the same 

 granite where it is intrusive in the possibly related Mundeti series 

 presently to be described. 



(the question whether we can admit the Idar granite masses 

 as "also responsible for the general crystalline condition of the main 



