ORIGIN OF THE GRANOPHYRE STRUCTURE. 171 



Its Origin. — The question of the origin of the granophyre in these doubt- 

 less clastic rocks is a difficult one. It is a question that deserves further 

 study, and I hope later to be able to throw a little more light on the 

 problem. For the present it can be said that the relation of the inter- 

 grown quartz to the quartz outside, as exhibited in a number of sections 

 lends no support to the view that the granophyre structure has been pre- 

 served from some detrital grains which have an igneous origin, but that 

 it is of a secondary nature, being developed in the already formed rock. 

 The usual interpretation of granophyre structure to indicate an igneous 

 origin for the rock in which it occurs is no longer tenable. Irving in 

 1883* described micropegmatite in a number of granitic porphyries 

 and augite syenites from Lake Superior, the secondary nature of which 

 was evident from its quartz being oriented like the areas of secondary 

 quartz lying without the feldspars. Sornewhat similar cases have been 

 described by Judd.f In a recent memoir by Julius Romberg^ on the 

 petrographic characters of an extensive series of Argentine granites the 

 granophyric structures are described in much detail with the aid of beau- 

 tiful plates. The author raises the question of their secondary origin 

 through weathering, and adduces many facts which make it probable 

 that this is their origin. 



In some of the feldspars which show the mottled structure in the 

 rocks now under consideration it can be determined that the inclosed 

 areas are feldspar of a somewhat different composition (microperthite 

 structure). This is particularly well shown in the instance of a feldspar 

 core having a rim which polarizes yellow, the core polarizing gray. A 

 set of fhottlings in the core give the same yellow tint as the rim and 

 extinguish with it. This would seem to show that the original feldspar 

 core had been partially replaced by a feldspar of different composition, 

 which composes the rim entire. This view is quite in harmony with 

 Wolff's deductions concerning the feldspars in the conglomerate schist 

 of Hoosac mountain.|| 



Secondary Enlargements of Feldspar. — Secondary enlargements of feld- 

 spar seem to be quite common in the rocks under investigation, and they 

 have been found at localities widely separated. § Occasionally these en- 



*The Copper-bearing Rocks of Lake Superior: Monograph V, U. S. Geol. Survey, 1883, p. 114, 

 plates xiv, xv. 



f On the Growth of Crystals in Igneous Rocks after their Consolidation : Quart. Journ. Geol. Soc. 

 vol. xlv, 1889, pp. 175-18G. pi. vii. Ibid., vol. xlii, 1886, p. 72. 



X Romberg, Petrographische Untersuchungen an argentinisehen Graniten,.mit besonderer Be- 

 rucksichtigung ihrer Structur und der Entstehung derselben : Neues Jahrbuch f. Mineralogie, 

 etc., Beilage-Band viii, 1892, pp. 314-323, 374-378, plates ix-xii. 



!| Loe. cit. See also Lehmann, Jahresbericht der Schlesischen Gesellschaft fi'ir Vaterlandische 

 Cultur, 1880, pp. 119-120. 



<j Thorpe mountain (numbers 3574, 347G ana 3477); northeastern slope of Mount Washington, 

 (number 2129) ; southeastern slope of Mount Washington (number 3104 ) ; Miles mountain (number 

 3213£), ami near Rattlesnake hill (number 3030). 



