262 PROCEEDINGS OP THE AMERICAN ACADEMY 



individualized ; for, where we find any of tliese in contact witli the 

 pyroxene, we see that the latter has adapted itself to the already 

 defined contours of the others. AYliile the pyroxene enclosed the 

 feldspar crystals with ease, it crowded the other constituents almost 

 wholly into the surrounding spaces ; a process which was facilitated by 

 the presence of the then fluid, uuindividualized substance. 



Tlie pyroxene contains, also, brown interpositions, similar to those 

 in the feldspar, and some opacite ; and, where it occurs with olivine, 

 it often surrounds the grains of this mineral. 



The magnetite bodies are of irregular shape, moulding themselves 

 sharply around the contours of the feldspar and olivine. Their sharply 

 defined outlines are black and fresh. 



The olivine is abundant in integral and aggregated grains, and, very 

 rarely, in crystals with recognizable though rounded contours. The 

 individuals are .0008 to .005 inch in diameter, and are readily distin- 

 guishable from the augite, between crossed nicols, by the difference of 

 colors, and in the crystals, by the parallelism of the principal section 

 with the longer sides. Where pains have been taken to give a tolera- 

 bly good polish to the surfaces of the thin section, the characteristic 

 finely pitted surface of the olivine distinguishes it, even in ordinary 

 light, from the augite, which takes a more perfect surface. 



There are few grains of olivine in this rock that are not more or 

 less altered to a very pale green substance, sometimes tinged with 

 brown. Under a high power, the olivine is seen to be traversed by 

 intercommunicating canals .0002 to .00005 inch thick, filled with a 

 clear, faint yellowish green to greenish-blue substance. From the sides 

 of these channels, jagged points of the same substance penetrate the 

 fresh olivine. In this manner, larger or smaller parts of tlie grains 

 have been changed to a feebly double-refracting substance which gives 

 an aggregate polarization due to the arrangement of the minute individ- 

 uals of the alteration product, which are sometimes felted, at others, 

 parallel fibrous. This product is dichroitic; pale green when the 

 fibrous structure is parallel to the shorter diagonal of the polarizer, and 

 pale orange when parallel to the longer diagonal. On the uncovered 

 sections, this alteration product was found to be very soft under the 

 needle. 



Apparently, not more than twenty to thirty per cent, of the olivine 

 is altered, which is very remarkable in a rock of such great age, con- 

 sidering the fact, which is emphasized by Zirkel,* that the olivine is 



* Mikrosc. Besch. d. Mineralieu u. Gesteine, p. 217. 



