ERUPTI\'E ROCKS FROM MONTANA. 57 



XV, loth Census) as double -refracting analcite. When 

 isolated, it has the specific gravity of analcite, and, accord- 

 ing to an analysis of impure material, a similar composi- 

 tion, although the percentage of silica is too low. No 

 chlorine or sulphur. Sp. gravity: 2.24. 



It should be noticed that the rocks in which this variety 

 occurs are perfectly fresh, even more so than those con- 

 taining the isotropic mineral; the olivine and the augite 

 show no trace of decomposition. 



In the analcite basalts, as described here, there is no 

 evidence of decomposition, except that the olivine is occa- 

 sionally converted to yellowish -brown serpentine. In 

 other specimens, however, it is seen that the analcite offers 

 but slight resistance to decomposition ; needles of a zeo- 

 lite with vivid colors of interference, probably stilbite, pen- 

 etrate the analcite in all directions and soon every crystal 

 is transformed to an aggregate of zeolites. Large stilbite 

 crystals are found in the decomposing rock. The augite 

 is much more resistant and frequently remains intactwhen 

 all the other constituents have been entirely decomposed. 



2d See.. Vol. III. ( 6 ) May 28. 1800. 



