O H A P T E K IV. 



The basalt that has been erupted in the vicinity of Riohmoiul IMouiitaiu and to 

 the east, foruiing' Basalt Peak, Strahlenberg, and Crater Cone, and also that found iu 

 the ueitihborhood of Pinto, though varying much in macroscopical habit, that is, iu 

 eolor, density, and compactness, and in its occurrence in large masses or thinly fissile 

 ])la tes, exhibits in thin sections iiuder the microscope the greatest uniformity in struc- 

 ture and in the microscopical character of its component minerals. It has many 

 points of similarity to pyroxeneandesite and the grounds for its determination as 

 basalt will be considered when the nature of its elements has been described. In 

 general it consists of a very homogeneous mixture of lath-shaped feldspar microlites 

 and augite crystals and grains, the latter being in excess, with a smaller amount of 

 hypersthene, besides which are minute crystals of magnetite iu a more or less abundant 

 glass base. The whole is of very even grain and macroscopic phenocrysta are almost 

 never met with. Olivine, which is considered an essential constituent inmost basalts, 

 l)lays the part of a very inconstant accessory mineral in this variety. Since the micro- 

 scopical habit of the different minerals in the basalts from the above mentioned 

 localities is constant throughout the series of thin sections, a single detailed descrip- 

 tion of them will be sufficient. 



The feldspar is tricliuic, in lath-shaped crystals for the most part well developed; 

 their size varies considerably within certain limits, the average length being between 

 0-1 and 0-05 """, a few reaching 0-25 """, and a nnich greater number being microscopic- 

 ally minute. They have a sharp outline along the base and brachypinacoid, but are 

 less regularly terminated, partly squared oft' as if by a x)inacoidal face; they are gener- 

 ally notched or pronged, apj)earing as if made up of several i>risms of unequal length ; 

 frequently the halves of a twin are separated for a short distance at either end of the 

 crystal by a tilm of globulitic glass. The smaller individuals show but two twinning 

 stripes, but in the stouter crystals more are present. They are of different lengths, 

 sometimes wedging out in the middle of a crystal. A second twinning at nearly 90° 

 to the first is seldom seen, except i;i some of the stouter individuals. In all the thin 

 sections where the feldspar microlites are of sufficient size, the angles of extinction 

 reach those of anoi-thite for many individuals, which also show a very high light in 

 extremely tliin sections between crossed nicols, indicating that a portion of the feld- 

 spar belongs to that species. There are besides many more faintly polarizing crystals 

 386 



