MUSEUM OF COMPARATIVE 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 Lévy? 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 which 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 with a modification 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 chlorite-like 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 through 
. 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 Minéralogie Micrographique, p. 227. 
8 Archiv der Naturw. Landesdurchforschung von Béhmen, III. Band, 5 Abth., 
Prag, 1877. 
* Mikrochemische Methoden zur Mineral Analyse, Amsterdam, 1881. 
