Van Hise — Enlargements of Hornblende Fragments. 233 



mineral constituents. However in these cases there are always 

 present coarser grains of hornblende and feldspar, and the 

 former commonly show enlargements. 



The following is a brief description of one of the thin sec- 

 tions from the Ogishke Muncie rock, the mineral constituents 

 of which are in the main quite clearly defined. The section is 

 composed of a rather sparse fine ground-mass, through which 

 are abundantly scattered medium-sized particles of hornblende 

 and feldspar. Although cut from a conglomerate, at a first 

 glance the section somewhat resembles that of a basic por- 

 phyry, but upon a closer examination its fragmental charac- 

 ter is plain. With a high power, the ground-mass, which 

 makes perhaps one- fourth of the section, appears to be com 

 posed of quartz and feldspar, with some kaolin and crypto- 

 crystalline silica. The other four-fifths of the section are com- 

 posed almost wholly of medium-sized grains of feldspar and 

 hornblende in about equal proportions. The feldspar is in 

 part orthoclase and in part plagioclase, the grains being usually 

 more or less rounded and varying in size from those several 

 millimeters across to those so small as to be lost in the fine 

 matrix. Some of these grains of feldspar have apparently 

 received enlargements. The hornblende is green or greenish 

 yellow in color. In most cases it shows one cleavage and fre- 

 quently is so cut as to develop nicely its two cleavages at the 

 characteristic angle. In unpolarized light these individuals 

 appear to be bounded by well defined lines, their apparent 

 forms being ordinarily oval, rounded or rectangular. Much 

 more rarely they possess crystalline outlines. In the polarized 

 light, however, each grain of hornblende is found to extend 

 beyond its apparent outline, as seen in the ordinary light, and 

 to terminate commonly in a ragged outline with long projec- 

 tions. These present outlines are believed to be due to en- 

 largements of the hornblende, the inner rounded, rounded 

 rectangular, or crystalline outlines, bounding fragments of 

 hornblende which after being deposited in the positions now 

 occupied have taken a new growth. The lines of division 

 between the fragmental cores and the exterior parts are dis- 

 tinct. They are due to numerous gas cavities and inclusions, 

 the latter being mostly minute particles of ferrite, i. e. these 

 lines of division are of the same nature as the corresponding 

 lines in the enlargements of quartz fragments. When viewed 

 in unpolarized light, in many cases there is little difference in 

 the appearance of the cores and exterior portions of hornblende 

 while frequently there is a marked difference. The added 

 portions are paler in color, at times so much so as to give the 

 impression that this part is quite different from the cores; yet, 



Am. Jour. Sci. — Third Series, Vol. XXX, No. 177. — Sept., 1885. 

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