OF ARTS AND SCIENCES. 265 



with muriatic acid, the discoloration is removed. The staining is, 

 therefoi'e, probably due to a deposition of the oxide in the cleavage 

 planes. 



Under the microscope, the cleavage is very evident in many individ- 

 uals, but it is more marked in the less altered rock. In those instances 

 where the cleavage is well defined, an examination with one nicol show 

 decided absorption, — the substance being almost colorless when the 

 cleavage lines coincide with the longer diagonal of the nicol, and delicate 

 green when perpendicular to this. 



Between crossed nicols, darkness occurs when the cleavage lines coin- 

 cide with a nicol-plane ; and some individuals which show no cleavage 

 lines remain dark during a revolution. 



This should seem to indicate that the substance is uniaxial. 



Many of these grains, especially in the more altered rock, differ from 

 the above in that they showan aggregate polarization due to a confused 

 fibrous or felted structure. 



The pyroxene in the more altered rock is apparently wholly changed 

 to an imjiellucid white substance, which, between crossed nicols under 

 a high power, shows a brilliantly variegated aggregate polarization, an 

 iiregular damask arrangement of bright-colored lines, forming concen- 

 tric circles or parallel wave-like lines. This substance is not confined 

 wholly to the pyroxene : it occurs also in the form of irregular veins 

 which have a rougii tendency to parallelism, and no defined walls but 

 which extend through the feldspar crystals as well as through the other 

 constituents. 



One of the specimens of this rock has at one end fragments of 

 the Mabb's vein in the shape of five or six lenticular layers of calcite, 

 with quartz j\ — ^ inch thick, separated by still thinner layers of the 

 rock. A close examination of the specimen near this portion reveals 

 the presence of very minute veinlets of calcite parallel to these larger 

 ones, while a still closer observation of the weathered surface shows 

 that still finer parallel veinlets occur throughout the specimen. 



Under the microscope it became evident that some of the veinlets 

 contained quartz as well as the calcite. 



On etching a thin section with acetic acid under the microscope, the 

 removal of the calcite was observed, and it became evident that the 

 irregular veins of impellucid white substance (which I mentioned as 

 extending beyond the pyroxene crystals) followed these veinlets, the 

 calcite forming the median line of the impellucid veins. The acetic 

 acid removed the calcite through to the under side of the section, but 

 the impellucid substance remained ; nor was it appreciably changed by 



