256 THE CRYSTAL FALLS lEON-BEAKlNG DISTRICT. 



the specimens collected and studied by him. I can obtain no positive proof 

 for or ao-ainst this statement. If it is an orthorhombic pyroxene, it agrees 

 with the inner zones of related occurrences wliich have been described by 

 Tornebohm, G. H. Williams, Adams,^ Romberg,^ and others. This zone is 

 at any rate composed of a colorless, compact mineral, with high single and 

 moderately high double refraction. Its single refraction is nearlj^ equal 

 to that of olivine. The mineral, as a rule, extinguishes parallel to the lines 

 of cleavage. In a few instances the line of extinction made a scarcely 

 noticeable angle with the cleavage. It is separated from the olivine by a 

 sharp line. At times this inner zone seems to disappear, and at others 

 becomes considerably broader than the average. The width is usually 

 about 0.02 mm., though it becomes at times 0.08 mm. 



Outside of this pyroxene zone there is a very much broader zone of 

 light-green hornblende. This is compact, and is in optical continuity with 

 the ordinary brown hornblende, which is the dominant hornblende in the 

 rock. This, in its compact nature and in its relation to the compact brown 

 hornblende of the rest of the slide, differs from the short fibrous actinolite 

 zone ordinarily described as taking part in such "reaction rims." This 

 hornblende zone reaches an extreme width of 0.15 mm. The outer edge of 

 this zone is penetrated by tubular ramifying growths of a colorless mineral, 

 which usually extend inward, perpendicular to the periphery, and which 

 appear to be continuous with the feldspar. This portion of the hornblende 

 rim is about 0.05 mm. wide No such intergrowth of feldspar with the 

 brown hornblende was found, nor have I been able to find elsewhere any 

 description of such an outside zone.^ However, Romberg describes the 

 interesting occurrence in an olivine-gabbro from the Argentine Republic 

 of zones around the hornblende which are very much like those above 

 described, except that the pseudopodia-like growths, as he describes them, 

 consist of a dark-green spinel instead of a clear white feldspar, as in the 

 Michigan rock. 



In some cases, where the olivine and augite are in juxtaposition, the 

 inner orthorhombic pyroxene zone completely surrounds the olivine. The 

 outer hornblende zone, however, surrounds both the augite and the olivine 



'Uberdas Norian oder Ober-Laurentian von Canada, by F. D. Adams: Neues, Jahrbuch fiir 

 Mineral, BB. VIII, 1893, p. 466, where references to observations made previous to 1893 may be found. 

 - Op. cit., p. 322. ' Op. cit., p. 323. 



