784 ABSTRACTS OF PAPERS 



are changed permanently. These factors can be and are being determined by 

 modern laboratory methods, and are in turn directly applicable to the study 

 of rocks. The data now available on the geologic thermometer scale indicate 

 that the establishment of such a scale is feasible, and can be accomplished by 

 a sufiBcient number of proper laboratory determinations. 



Discussion 



Prof. J. F. Kemp: Mr. Wright's ingenious application of the peculiar prop- 

 erties of quartz will be of great service in testing some of our conclusions 

 hitherto reached regarding the bodies of magnetic iron ore in the ancient 

 gneisses. In later years observers have more and more drifted away from 

 the sedimentary conceptions of origin and have proved a direct crystallization 

 from fusion. In the Adirondacks and elsewhere the ores are in quartzose 

 rocks. If now the quartzes have been formed below 575" C, fusion, unless 

 followed by complete metamorphism, is out of the question. But if the 

 quartzes show the properties of the higher temperature the hypothesis of 

 development from fusion will be corroborated. Mr. Wright has already con- 

 sented to make some tests which will be of much significance. 



OBSIDIAN FROM HRAFNTINNUHYGGUR, ICELAND 

 BY FRED E. WRIGHT 



iAl)stract) 



Describes (a) peculiarly pitted surfaces on specimens of obsidian which 

 resemble in a remarkable degree the markings of the Austrian moldavites; 

 (&) also a unique type of crystallization in cavities in this obsidian. 



PEGMATITE IN THE GRANITE OF QUINCY, MASSACHUSETTS 

 BY C. H. WARREN AND CHARLES PALACHE 



(Abstract) 



Only two important occurrences of pegmatite are known in the riebeckite 

 granite of Quincy. These are exposed in two of the quarries and take the 

 form of rudely cylindrical masses of considerable size entirely inclosed in the 

 granite. In mineral composition they are closely similar to the granite, the 

 essential minerals being quartz, alkali-feldspar, riebeckite, and segirine; acces- 

 sory minerals so far identified are fluorite, parisite, octahedrite, ilmenite, 

 wulfenite, and the sulfides molybdenite, galena, sphalerite, and chalcopyrite. 

 The pegmatites exhibit a certain symmetry of structure. Fine graphic-granite 

 forms a marginal band, succeeded centrally by a zone of coarse granitic text- 

 ure made up of quartz, feldspar, riebeckite, and segirine-augite. As a rule, this 

 zone graduates centrally into almost pure massive quartz, sometimes contain- 

 ing sulfides. In one portion of the largest mass the center is microlitic, and 

 in the cavities thus formed the quartz, feldspar, and aegirine are well crystal- 

 lized, while the rarer minerals noted above find there their principal develop- 

 ment. Angular fragments of the pegmatite, inclosed in felted crocidolite, and 

 deeply corroded crystals of riebeckite partly replaced by fluorite, point to a 

 final stage of crushing and pneumatolytic action. The paper describes these 

 deposits and the minerals in detail. 



