AKT. 28 MINERALOGY OF TRIASSIC LIMESTONE SHANNON 13 



coarser varieties of pectolite. The feel is liarsli and needles break 

 off and enter the fingers as splinters like pectolite. The xonotlite 

 is mixed with calcite and some of the fissures have an earlier layer 

 of datolite next the wall. The needles form radiating bundles and 

 rosettes on the crack-surfaces, sometimes 3 cm. across. Under the 

 microscope these lie on a perfect cleavage which is probably per- 

 pendicular to the obtuse bisectrix. If this be taken as &(010) the 

 optical orientation is Xr=5, ¥=«, Z=c. The elongation of the 

 needles is positive and they give parallel extinction. The mineral 

 is biaxial positive with 2V probably small. The refractive indices 

 are ar=1.583, ^=1.583, 7=1.595. The material gave the following 

 composition upon analysis : 



Analysis of coarse xonotlite 



SiOo 49.60 



AI2O3 1. 00 



CaO ■- 46. 32 



HoO+110°C 2. 80 



H2O— 110°C None. 



Total 99. 72 



THA0MASITE 



Certain glassy transparent grains making up about 1 per cent of 

 the first analyzed sample of xonotlite were not fibrous, had a fairly 

 high birefringence and were uniaxial negative with (.i=rl.505 and e 

 decidedly lower. These, to judge from their optical properties, were 

 probably thaumasite. The analyzed sample gave faint qualitative 

 reactions for carbonic and sulphuric acids. 



WOLLASTOXITB 



Small interstitial areas between the masses of xonotlite are filled 

 with a glistening material of fine bladed structure varying in color 

 from pearly white to pale greenish. This material, when powdered 

 and examined under the microscope, yields laths with parallel ex- 

 tinction, biaxial negative, 2V small, r<'y weak, Y=elongation, ^ 

 above 1.62. This is doubtless wollastonite. It is difficultly distin- 

 guishable, with the naked eye, from the crystalline calcite of the 

 marbles and, although none was found on a later visit to the quarry, 

 the mineral may not be uncommon. 



DISCUSSION 



The foregoing descriptions may now be summarized. Magmatic 

 solutions, emanating from diabase, have traversed fissures penetrat- 

 ing overlying limestone and have largely replaced the rock adjacent 



