«it 
ORTHOCLASE-FREE GABBRO. 39 
Wisconsin, as first shown by Julien,’ the olivine shows a very interesting 
and unusual mode of change, namely, into biotite, viridite, and tale, the 
two former replacing the interior of the olivine grain, the latter forming 
a sort of shell of minute flakes around the outer part of the grain. Parti- 
cles may be seen in all stages of this change. 
Next to the olivine, in order of age, is the plagioclastic ingredient. Its 
crystals are usually in elongated forms, running from under a sixteenth of 
an inch in length to two or three inches in the coarsest kinds. The outlines 
of these crystals are commonly linear, or at least partly so, but in some 
sections the mutual interruptions have produced completely rounded con- 
tours. In composition—to judge from measurements of the angle contained 
between the maximum extinction positions of adjacent hemitropic bands, in 
sections cut at random in the zone OQ: 7%, as viewed between the crossed 
nicols—the plagioclase appears always to be near the basic end of the feld- 
spar series. The sections measured were known to be in this zone by their 
giving equal, or nearly equal, angles between the position of coincidence 
of a nicol plane and the lines of junction of the hemitropic bands, and the 
maximum extinction position of the one set of hemitropic bands on the one 
side of this position of coincidence and that of the other set on the other 
side. The method is Pumpelly’s modification of Des Cloizeaux’s method 
of distinguishing between the plagioclase feldspars.” According to it, those 
feldspars from which the largest angles obtainable, after measuring a number 
of individuals, are below 36°, are classed as oligoclase; those whose largest 
angles lie between 36° and 62°, as labradorite; and those giving angles 
above 62°, as anorthite—the size of the angles increasing with the basicity.’ 
1Vol. III, Geology of Wisconsin, p. 235. 
2 Metasomatic Development of the Copper- bearing Rocks of Lake Superior; Proc. Am. Acad. Sci., 
Vol. XIII, pp. 30, 31. (1878.) 
3The method is, of course, open to the objection that it determines the presence of only the high- 
angled or basic feldspars. In a rock of which a number of sections refused always to give any but low 
angles this would not be any objection, but in those giving high angles it leaves room for doubt. Nev- 
ertheless, in the present case I have little doubt that there is only one feldspar concerned. This is 
indicated by the exact similarity in all respects in any one thin section between the high-angled and 
low-angled particles, the latter of which are, indeed, but few in number. So far as the Lake Superior 
rocks are concerned, I have always found those feldspars which refuse, through a number of sections, 
to give high angles to have strongly-marked peculiarities, among which greater tendency to decompo- 
sition, and relatively very narrow lineations between the nicols are most prominent. It is also my 
experience that these low-angled feldspars are always associated with orthoclase, or, if they occur with- 
out orthoclase, that they are only large-sized porphyritic ingredients in an aphanitic base. 
