200 BASALT. 



also has a cleavage wLicli is often well marked, and divides the mineral into rhomhic 

 forms, with angles, as approximately determined by several measurements, of 73° and 107°. 

 The principal cleavage is parallel to the longer diagonal of tliese rhombs. It is tliis 

 rhombic cleavage, probably, which has led observers to believe that chladuite crystallized 

 in the monoclinic and triclinic systems. 



The enstatite is found to contain many glass inclusions with polyhedral outlines, the 

 planes being presumably, as usual in such cases, the planes of the inclosing mineral. While 

 many of these inclusions are arranged in the enstatite parallel to the cleavage planes, 

 others are placed at every angle with those planes. The glass inclusions carry bubbles, 

 microlites, and rounded lenticular forms. The latter are frequently at the end of the 

 inclusion, and in some cases, show the cherry-brown color of some chromite. This mate- 

 rial, besides forming inclusions in the glass, is in lenticular and irregular rounded grains 

 in the enstatite itself. It sometimes extends in a series of grains across the entire enstatite 

 mass, and at others is in isolated forms. These inclusions, microscopically, are seen to be 

 composed of a centre of nickeliferous iron or pyrrhotite, surrounded by a band of dark 

 material, — chromite or magnetite, possibly. These ferruginous materials are in many 

 cases surrounded by a yellowish-brown staining of iron, which sometimes extends over 

 considerable of the mass and along the fissures. Numerous vacuum or vapor cavities were 

 observed, which were arranged in one plane of the enstatite. The inclusions are seen to 

 be crossed and cut by the cleavage and fissure planes of the enstatite, showing that they 

 were of prior origin to the fissures. 



The feldspar stands next in abundance to the enstatite, and is in irregular masses held 

 in its interspaces. It is water-clear, and almost invisible by common light. Much of it 

 is seen to be plagioclastic, but the twinning bands are so exceedingly fine, and the polar- 

 ization colors so bright, it does not, as a rule, show well this character, except with high 

 powers, and when the mineral is near tlie point of extinction. 



The feldspars contain numerous yellowisli-brown, dark, and almost colorless inclusions, 

 which are sometimes irregularly scattered, but more commonly are arranged in bands, 

 similar to those of the fluid inclusions in c[uartz. These glass inclusions are of various 

 dimensions, and many contain a small bubble. Some microlites were also seen. 



In the feldspar at one end of a section, the enstatite was found in minute crystals ex- 

 tending outward from a centre, forming stellate or rosette-like forms. The structure is like 

 that oliserved in terrestrial rocks, in minerals formed from alteration or solution. This 

 apparently might have been produced in this case, either by the rapid crystallization of 

 enstatite material in a liquid feldspathic mass, or by secondary alteration through water- 

 action on the rock itself. The absence of any other signs of alteration, except of the 

 ferruginous materials, seems to negative the latter supposition. The ferruginous alteration 

 can probably be accounted for by the absorption of moisture by this friable fissured stone 

 since it reached the earth. 



The bands of inclusions were seen in several instances to extend from the feldspar 

 through the enstatite, and in one case, to pass into another feldspar on the opposite side. 

 This indicates that the cause of these inclusions was a general one for the rock-mass, and 

 not limited to any one mineral. Enstatite was found in a few cases inclosed in the 

 feldspar. 



The monoclinic pyroxene or augite is less abundant, and its determination less sure 

 than is the case with the enstatite and feldspar. It is crossed by fissures in a very irreg- 

 ular manner, but shows in some cases the approximately right-angled cleavage of augite. 

 Its optical cliaracters appear to be those of that mineral, but its polarization is more bril- 



