BAIN.] 



MINES AND PBOSFECTS. 57 



the presence of minute grains and crystals of calcite and quartz. The 

 bedding may still be made out, but the outlines of the calcite crystals 

 are completely obscured. The very intimate association of the calcite 

 and fluorite are notable and indicate the entire penetration of the rock 

 by the fluorine. The calcite of the large crystal in the lower portion 

 of the field has been partly replaced by fluorite, the bounding planes 

 being evidently determined by the cleavage planes of the calcite. The 

 calcite is under strain, as is indicated by the undulatory extinction 



In C a view is shown, in natural light, of the central portion of 

 one of the bands of fluorite. Eunning through this band is an irregu- 

 lar plane, which might readily be mistaken for such a surface as is 

 l^roduced by the interlocking of crystals making up the comb struc- 

 ture of ordinary banded ore deposits. In this case, however, the 

 irregularities of the line bear no relation to the optical orientation of 

 the crystals, and the line is furthermore of the exact character com- 

 mon in sedimentary beds betw^een individual layers, particularly 

 where the beds have been under slight pressure, so as to produce 

 stjdolites. 



The above evidence warrants the conclusion that in this ore body 

 the fluorine has been introduced along horizontal bedding planes and 

 that fluorite has metasomatically replaced the calcite of the limestone, 

 producing a banded structure without crystallization in free spaces. 

 On the whole it forms an interesting and typical example of the 

 phenomenon of pseudo-crustification, discussed some years ago by 

 Mr. Emmons." In this case horizontal bedding planes rather than 

 vertical shearing planes are concerned. 



With the metasomatic replacement of the rock there has been also 

 some free growth of crystals wherever space has been available in 

 druses or the brecciated rock. Very beautiful specimens of golden, 

 purple, and white fluorspar occur, showing the cube with occasional 

 modifications by octahedral faces. 



The introduction of the fluorine solutions was doubtless from the 

 main fault along the foot of the hill, and occurred before the develop- 

 ment of the present topography. The calcareous sandstone forming 

 the Rosiclare member of the limestone was the particular localizing 

 agent, as it forms a firm roof over the mine, and, being of unusual 

 thickness, 25 feet, was a competent stratum under mild stresses. 

 This had the effect of opening up the upper bedding planes of the 

 limestone and allowing slight local brecciation, so as to afford entrance 

 to the solutions. 



The main fault at the foot of the hill is complicated by the presence 

 of a number of minor faults. Along one of these near the mine is an 



" structural features of ore deposits : Trans. Am. Inst. Mln. Eng., vol. 16, 1888, pp. 

 804-839. 



