570 Professor Emerson on HalVs Geological Collections. 



broad conclioidal fracture, and when raoistened slightly mottled with green and 

 brown. It api)ears quite fresh in the interior, but eflervesees with acids. On the 

 outside is a light reddish-gray layer of decomposition 2 mm. thick. On the fresh 

 surface it has a glistening luster, and with the lens one detects minute scales of 

 rubellan, fine needles of hornblende, and roundish spots of reddish color, which 

 prove to be garnets. A few small cavities have been filled with a whitish mixture 

 of calcite and a zeolite, in which radiating needles of shining black secondary horn- 

 blende appear. The rock aftbrds a reddish-gray powder and fuses at 3 to a whit- 

 ish enamel. The powder treated with acid and examined under the microscope 

 shows only slight changes ; a small quantity of ocher and calcite is removed. 

 Examined in thin section, the rock is found to contain the following minerals: 

 Garnet, which makes up more than half the mass; biotite next in abundance; 

 then hornblende and \iridite, and more rarely apatite, hematite, calcite, magne- 

 tite, and a zeolite. Cyanite occurs in long flat crystals, transparent, brilliant, 

 cut across by broad cleavage fissures filled with viridite. It polarizes with great 

 beauty; occurs commonly in mica. The garnets are scattered through the whole 

 mass, gathered in small groups or occurring singly, separated from each other by 

 mica and hornblende. Many large garnets occur also in the mica. They occur 

 mostly in rounded grains up to .45 mm. in diameter. Imperfect four, six, and eight 

 sided cross- sections are not rare, and the smaller crystals inclosed in the larger 

 and especially in the mica are often perfect rhombic dodecahedrons. The sec- 

 tions of the crystals appear moderately magnified a pale reddish-brown to Isa- 

 bella-yellow, being more or less clouded with a brown dust, except at the narrow 

 border, where they are quite pellucid and colorless or show a faint tinge of violet. 

 The transparent portions are stiU isotrope, and the central portions show aggTe- 

 gate polarization, but when highly magnified it is seen that the transparent gar- 

 net substance predominates in most crystals. In a few cases the decomposition 

 proceeds from the circumference, and the center is still quite transparent. The 

 dust is made up of chlorite plates, blood-red hematite scales, and amorphous 

 grains. 



Many smaU perfect dodecahedra inclosed in the larger garnets are of bright 

 lemon-yellow color, and show distinctly cleavage after oo O. Thej' are of the 

 same yellow color throughout, and show no tendency to the accumulation of 

 granular matter at the center. Many — apparently hornblende — microlites, ar- 

 ranged in an irregular net- work, are found in them, sometimes passing out into 

 the surrounding mica. 



Ifext in abundance is rubellan, in broad, fresh, transparent plates, bright 

 hyacinth red to deep blood red, separating the groups of garnets from one 



