MUSEUM OF COMPAEATIVE ZOOLOGY. 5 



present. The feldspar is shown by twinning striations to be plagioclase, 

 which exhibits when fresh beautiful zonal phenomena. In a number of 

 sections the method of Professor Pumpelly ^ or M. Michel Levy ^ was 

 applied for the determination of this feldspar. As is well known, this 

 method consists simply in a determination of the maximum extinction 

 angle in the zone of the macro-pinacoid and base, sections whicli are in 

 this zone being characterized by symmetrical positions of extinction in 

 the two sets of twins, with reference to the twinning plane. Results 

 were obtained as high as 27°, requiring the presence of a feldspar as 

 basic as labradorite. In the classical work of Pumpelly above cited, 

 crystals of feldspar from the Granite Street locality were determined by 

 this method, combined witli a modihcation of Des Cloiseaux's method 

 for determining the size of the basal extinction angle. The highest re- 

 sult obtained by the first method was 16°, and by the second 3° to 4°, 

 though, owing to the sections being inclined to the base, the latter re- 

 sults were more or less unreliable. He concluded that the feldspar was 

 probably albite or oligoclase. 



Mechanical separations of the constituent minerals have been made in 

 a number of cases by the Thoulet solution. In every case feldspar was 

 removed with each separation between the specific gravity limits 2.76 

 and 2.6, and often a considerable portion came below the inferior limit. 

 The grains were found to be seldom pure, and the wide range in specific 

 gravity is doubtless, in part, to be referred to decomposition products. 

 The portion separated below the limit 2.6 was in several cases subjected 

 to microchemical tests by both Boricky's ^ and Behrens's^ methods, after 

 careful washing to remove all iodide of potassium. Potassium as well 

 as calcium being always detected in this powder, that derived from No. 

 222 was subjected to quantitative chemical determination, which yielded 

 4.16% of oxide of potassium. The products of alteration of the feldspar 

 are calcite, and a mineral which is probably kaolin. Considerable 

 green chloritedike material is often contained in the feldspar grains ; but 

 it has apparently been derived from the biotite or augite by alteration, 

 and has found the way to its present position in the feldspar throiigli 

 the cleavage cracks. This substance is the viridite of Professor Wads- 

 worth, which he considered an incipient alteration of the feldspar. 



1 Metasomatic Development of the Copper-bearing Rocks of Lake Superior. 

 Proc. Am. Acad., XIII. 253. 



2 Mineralogie Micrographique, p. 227. 



3 Archiv der Naturw. Landesdurchforschung voa Bohmen, III. Band, 5 Abth., 

 Prag, 1877. 



* Mikrocliemische Methoden zur Mineral Analyse, Amsterdam, 1881. 



