Tdhle Mountii'irt, four (iohhiu Colorado. 135 



spherules of a similarly colored radiate mineral. These 

 spherules are seldom more than 2 mm in diameter, and are very 

 perfect spheres. They increase in number upward and finally 

 compose the greater part of the deposit. 



In one cavity, 6 to 8 feet in horizontal diameter and about 

 two feet in height, the deposit is quite different. Here the 

 main mass is loosely granular, and is formed chiefly by a 

 bright greenish-yellow mineral, while a stratified appearance is 

 produced by layers of a white or colorless mineral. Some of 

 the light layers are chiefly made up of easily recognizable 

 stilbite, and the same mineral in distinct tablets forms the 

 upper layer of the whole deposit. There are also irregular 

 seams of white running through the yellow mineral. 



The determination of the minerals in these deposits began 

 with the greenish-yellow sand of this last mentioned cavity, 

 and the description will follow the same order. 



If the loose sand be placed under the microscope with a 

 power of about fifteen diameters, it is seen to consist of pris- 

 matic grains, mostly with broken terminations. Many of the 

 grains are clear and transparent, with the greenish-yellow color 

 mentioned, while others are dull. The clear prisms polarize 

 strongly, and extinction takes place at an angle of 35° to 40° 

 with the vertical axis. 



On splitting open some of the white layers, surfaces are ob- 

 tained, showing minute, stout prismatic crystals, which, seen 

 under the microscope, present the same habitus as the yellow 

 grains. The prism angles are nearly 86° and 94°, and the 

 termination is usually formed by an oblique plane, like a 

 hemidome of the monoclinic system. The optical orientation 

 is the same as in the yellow crystals, and corresponds also to 

 the requirements of the monoclinic system so far, as can be 

 determined. The properties given agree with those of laumon- 

 tite, and in the light alyaes, the identifica- 



tion of both yellow and white crystals with that mineral seems 

 entirely justifiable. 



Oxygen i 



