E. II Kraus — Celestite- Bearing Rocks. 291 



liar porous structure of the leached rocks, and secondly, that 

 many celestite veins can be found crossing the various strata, 

 usually in a vertical direction. 



As already said, many of these leached rocks possess cavities 

 with outlines well enough preserved so that no doubt exists as 

 to what the mineral was which originally occupied them. The 

 best of such cavities, which have been studied thus far, are 

 usually quite small. In many instances cavities are' encoun- 

 tered which cannot be readily recognized as having been caused 

 by the solution of celestite. Some of these appear as though 

 the point of a knife had been thrust into the rock material 

 while in the process of hardening, and on this account the term 

 "gashed" dolomites has been used by Lane, Sherzer and 

 others. 



Sherzer* refers especially to such an occurrence at the Ida 

 quarries, likewise in Monroe County, but adds that such phe- 

 nomena can be seen throughout the whole Monroe series. In 

 fact, this peculiar structure has been observed in many places 

 in Michigan. Winchell in his report of 1860 calls it an " acicu- 

 lar " structure. He thought that gypsum w T as the original occu- 

 pant of the cavities. Later on, Rominger also used the term 

 acicular but made no definite statement as to what mineral had 

 occupied the cavities. Lane,f however, mentioned calcite as 

 possibly having been the original occupant. One of the diffi- 

 culties in recognizing the original occupant of such " gashed " 

 cavities has been due to the fact that the orthorhombic outline 

 is not always definitely preserved. Sherzer aptly describes them 

 as follows : " The rock looks as though, when it was only very 

 slightly plastic, it had been jabbed in every direction with a 

 thin-bladed, doubled-edged knife point. The gashes are always 

 open, intersect one another irregularly and vary greatly in size, 

 some being two-thirds of an inch long, while others can scarcely 

 be seen without the magnifier. The cross-section of each gash 

 shows that it is thickest at the center and that it slopes gradu- 

 ally and symmetrically to a very thin edge." Such an outline 

 can be readily referred to celestite, when we bear in mind that 

 crystals of this mineral are often found which have a tabular 

 habitus, that is, the basal pinacoid is predominant. Cross-sec- 

 tions through such tabular crystals would conform very closely 

 indeed to Sherzer's description above of the so-called " gashes." 



Another reason why the original occupant of these cavities 

 evaded detection lies in the fact, that celestite had not up to 

 the present time ever been noted as occurring in these rocks in 

 a disseminated condition, that is, in a condition of primary 

 origin. Therefore, with these facts before us, first, that celes- 



* Geological Eeport of Monroe County, 1900, 85. 

 flbid., foot-note, p. 86. 



