Table Mountain, near Golden, Colorado. 131 



a section placed in position II was wholly dark, excepting at one 

 of the angles of the square, where n faint light was visible, 

 producing distortion of the calcite interference cross at this 

 point. In another section, in position I, the dark figure was 

 not completely dark, there being light enough to admit of the 

 distinct appearance of two perfect black crosses, whose thick 

 bushy arms lay parallel to the diagonals of prism I, and which, 

 revolving with the section, disappeared entirely in position II. 

 All but transmitted light must be excluded, in order to see 

 these crosses distinctly, as the whole of the square is still very 

 dark in contrast with the outer zone. In cases where the 

 js of the dark figure are much-broken lines, the space 

 within is commonly divided into a number of irregular patches, 

 each with its black cross, seen in position I. In such cases, too, 

 the whole field does not become uniformly dark in any posi- 

 tion. The size of the dark figure relative to that of the section 

 varies greatly. In most cases the relation is similar to that of 

 fig. 1 ; while in some prismatic sections the dark square is 

 larger than can be inscribed within the prism, its angles being- 

 cut off by the outlines of i-i. Again, the dark figure becomes 

 very small, though in no case yet observed has it'been entirely 



It is impossible to indicate all the irregularities observed, 

 within the limits of this article, and the fuller description of 

 these interesting phenomena must be reserved for the final 

 report on the region embracing Table Mountain. 



None of the sections thus far prepared, parallel to the prism 

 i-i, have exhibited . n; marked abnormal properties. 



It is thought that the degree of variation, in the optical prop- 

 erties, from the simplest form illustrated by fig. 1, stands in 

 intimate relation to the degree of irregularity in crystal growth 

 indicated by the faceted surfaces. Certainly no hypothesis, 

 ■ ingenious, which considers the tetragonal symmetry of 

 esult of itrtr twin struct"" 



noclinic individuals itliat of Eumpf 1. a), ■ 

 explain the present case with a tithe of the plausibility w 

 diich the theory of inner tension* is able to do it. 



It is hoped that further investigations will prove the dir 

 pplicability of this latter theory to the present instance. 



a, p. 209. 



(b.) C. Klein. flea Boracits," ibi 



(c.) AJfredi : , - > -- '■ P- H- 



'■■'.. F. Klocko, ■ I.Vber Doppelbrechung regularen Kry-i.-'.i ■- ' i M- 



)ophyllit 



