12 PROCEEDINGS OF THE NATIONAL, MUSEUM tol. 73 



the mesolite fibers and is older than the stilbite of the same speci- 

 mens. It was found impossible to obtain pseudomesolite from the 

 specimens of mesolite in pure enough form for analysis. 



A second group of specimens of pseudomesolite consists of some- 

 what slaggy basalt coated with broad crusts of loose-textured 

 fibrous wliite material a centimeter or more thick, and containing 

 in this loose-textured fibrous material numerous more or less perfect 

 trapezohedral crystals of analcite a centimeter or so in diameter. 

 Optical examination showed the fibrous zeolite to be identical with 

 the pseudomesolite associated with mesolite, but entirely free from 

 mesolite. By avoiding the analcite and by scraping the best mate- 

 rial from the specimens, a sample pure enough for analysis was se- 

 cured. Under the microscope it had almost zero birefringence, 

 positive elongation, an extinction not measurable owing to extremely 

 low birefringence, and mean index of refraction 1.510 =t 0.002. The 

 material is easily soluble in hot 1 : 1 hydrochloric acid and gelatinizes. 

 The analysis gave the following results : 



Analysis and ratios of pseudomesolite 



Constituent : Per cent Ratios 



SiOj 43.80 0.726 0.242X3 



AI2O3 28.20 .276 .276X1 



CaO 10.48 0.1871 



MgO .04 .001 256 '>56X1 



K2O 1.46 .016| "^^^ .-&bXl 



NajO 3.22 .0521 



IIjO above 120° C 13.24 .735\ 707 oars^o 



H2O below 120° C .32 .002/ ' '"^' . -^dx^ 



100. 76 



The anal3?sis shows the material to be identical in composition 

 with mesolite, the only difference being in the optical properties- For 

 this reason it is designated pseudomesolite; a name was applied by 

 A. N. Winchell ^ to a zeolite of similar composition found in the 

 anorthosite of Carlton Peak, Minn. Pseudomesolite of Winchell 

 differs from normal mesolite in optical properties, being positive in 

 elongation, with an index of 1.5 and birefringence of 0.002. It has 

 inclined extinction up to 20°. Owing to the thinness of the fibers 

 here described and their very low birefringence no inclination of 

 extinction could be detected. 



A third mode of occurrence is in the form of tufts on thomsonite 

 or, in one case, calcite. 



By its position in several different types of association the pseudo- 

 mesolite is shown to be older than analcite and younger than thom- 

 sonite, mesolite, and calcite. It is more abundant and more widely 

 distributed than mesolite in the specimens from Ritter Hot Spring. 



•^Amer. Geologist, vol. 26. p. 275. 1910. 



