52 IOWA ACADEMY OF SCIENCES. 



be stated briefly. They are best set forth by Whitney* aud more recently, 

 and in greater detail, by Chamberlainf. 



First, then, as to the formation of the crevices. Extending east and west 

 through the zinc region are numerous and abrupt undulations of the strata. 

 These were caused by a horizontal pi'essure acting from the south resisted 

 by a corresponding force from the north. To state it differently, the oscil- 

 lations are due to lateral force from the Interior Sea to the south and 

 I'esisted by the Archa3an laud area to the north. These flexures pi'oduced 

 the crevices. As the strata were elevated the heavily bedded limestones 

 were fissured parallel to the axis of elevation and more or less open crevices 

 formed. In a direction at right angles little force was exerted and the beds 

 were only fractured, producing north and south Assures. 



As suggested by Whitney, the shrinkage of the rocks may account for 

 some of the crevices, at least to their open character, though it is difKcult 

 to see how shrinkage could have the great influence attributed to it by that 

 writer. 



The ore receptacles having been formed, whence caiue the zinc to fill 

 them? It will be necessary simply to mention hei'e the rejected hypotheses, 

 namely, those of sublimation, and of thermal waters. Facts ai'e well nigh 

 overwhelmingly against the idea that the fissures extend to any great 

 depth, being confined chiefly to the Galena aud Trenton limestones, and 

 without such extension downwards either of the above theories are very 

 improbable if not impossible. All the facts indicate that the zinc comes not 

 from below, but from the limestones in which occur the crevices. It was 

 deposited along with the sediments by the waters of the Silurian sea. The 

 latter derived its metallic salts from the waste of the pre-existing land sur- 

 faces. Chamberlain describes in detail the cause of localization of the 

 deposits to a few areas, ascribing it to the currents of the ancient sea, taken 

 in connection with the precipitating agencies of organic matter. 



After their deposition in the limestone beds the zinc was concentrated in 

 the crevices by the action of drainage waters percolating through the 

 metal-bearing beds. In this way the zinc was concentrated in the fissures 

 where it is now found. 



SATIN SPAR FKOM DUBUQUE. 



BY A. G. LEONARD. 



Located less than six miles south of Dubuque and one and three-fourth miles 

 due west of Massey station on the Chicago, Milwaukee & St. Paul Railroad are 

 some curious "spar caves" as they are appropriately called. In these caverns are 

 some occurrences of satin spar that are very unusual and of much interest. It is 



♦Geology Wisconsin, vol. I, 1862. 



+ Geology Wisconsin, vol. IV, 1873-1879. 



