652 FRANK F. GROUT 



is usually radial, but in some, confused and very fine grained; double 

 refraction is very slight and still notably variable. 



Probably second in importance as an alteration product in these 

 rocks are laumontite and related minerals. Their occurrence is 

 mentioned above and it is now time to discuss the evidence of the 

 relation of laumontite to an aggregate of quite different composition. 

 The laumontite crystals, occurring on Snake River in greatest abun- 

 dance, are red in color and usually form radiating groups up to two 

 inches across. Terminal faces have developed on distinct prisms 

 up to a quarter of an inch in thickness. The termination is the com- 

 mon laumontite form, classed as a dome (201) by Dana. The angle 

 from prism to this dome varied quite widely in the crystals measured, 

 but apparently this was due to the enlarging prism in the radiating 

 group. Several measurements, by both hand and reflection goniom- 

 eters, lay very close to the correct figure 113° 30' and the rest varied 

 within 5 degrees below that value, with no point more prominent 

 than the rest. A trace of the pinacoid face could be seen on a few 

 crystals. The prismatic cleavage is prominent, especially after the 

 mineral has stood a time in a dry warm room. The blowpipe and 

 physical characters clearly identified it. 



Thin sections indicate hematite as coloring matter rendering the 

 section dusty, but other alteration products contributed to this effect. 

 Extinction was often wavy, and inclusions of different orientation 

 crossed some grains in such a way as to remind one of microperthite. 

 Such was the material of Sample 2, Table VI. 



Light-green pseudomorphs formed from this red mineral. They 

 preserved the angles of the original and showed the same development 

 and imperfections. In a few illustrative specimens, the red center of 

 laumontite is visible in broken green prisms. The cleavage cracks of 

 laumontite show as light or more transparent lines in the pseudo- 

 morph. At this type locahty, the chief chemical change is the replace- 

 ment of lime and some water, by potash and magnesia. No formula 

 is yet suggested for the mineral, however, because of the variation in 

 similar material from other localities. It has a hardness of 3 ; specific 

 gravity of 2.7; fusibihty of 3 ; soapy feel; is an aggregate of grains, 

 none of which was over o . 01 mm. in length ; is anisotropic and has 

 medium birefringence. Hydrochloric acid dissolves very little, and 



