Minerals of the Composition Mg 'Si % . 409 



to the rapidity of the process, we were led to suspect what 

 afterwards proved true, that the amphibole would pass over 

 into the other with evolution of heat. This and the method 

 of its formation by sudden cooling are characteristic of mono- 

 tropic forms. 



Interesting results were obtained in an attempt to measure 

 the approximate temperature at which the amphibole crystal- 

 lizes. As a determination of a physical constant the attempt 

 failed, because the disturbance caused by touching the liquid 

 with the element was sufficient to start the crystallization of 

 the monoclinic form in every instance. The following trials 

 were made with the bare thermoelement dipped directly into 

 the liquid, which above 1100° is easily penetrable with a wire. 



Experiment 1. — As soon as crystallization began, the element was 

 dipped into the center of the charge. Temperature 1350°. 

 All monoclinic pyroxene. 



Experiment 2. — Amphibole started to crystallize in the outer 

 zone. Measurement as in 1. Temperature 1300°. Mono- 

 clinic pyroxene started at once. 



Experiment 3. — Same as 2. Temperature 1300°. 



Experiment 4. — Amphibole began to ciystallize on the upper 

 surface next the crucible wall as in 2. The element was 

 dipped in, on the boundary line, between solid and liquid, 

 about l cm from the crucible wall. Temperature 1150°. 

 Monoclinic pyroxene started instantly. 



Experiment 5. — Amphibole began to crystallize as usual. We 

 waited until it nearly covered the surface and then dipped 

 the element into the liquid, which remained in the middle. 

 Temperature 1180°. When the charge was removed from 

 the crucible, all but the upper layer proved to be mono- 

 clinic pyroxene. 



Experiment 6. — Like Experiment 4.. Temperature 1165°. 



The rise of temperature with crystallization, the rapid 

 radiation of heat from an unprotected crucible, the variation 

 of temperature in different parts of the mass, all prevent any- 

 thing better than rough temperature estimates, but the fact 

 that the amphibole forms in a region near which a slight 

 disturbance is sufficient to crystallize the glass to monoclinic 

 pyroxene, is conspicuous and important. This temperature has 

 been shown in an earlier part of the paper to be above 1150°. 



Properties of Orthorhomhic Amphibole. — This amphibole 

 appears white and porcelain-like, and crystallizes in radial 

 spherulites and fibrous aggregates. Since we have not yet been 

 able to obtain crystals suitable for measurement, its identifica- 

 tion rests solely on the correlation of its other properties, chiefly 

 optical, with those of the natural mineral, kupfferite. 



The fibers were too fine and too intimately intergrown to 



