80 PHYSICAL GEOGRAPHY. 



discovered in Iowa, bnt the water of artesian wells is 

 frequently fonnd charged with soluble mineral substances, as 

 will be seen in the report of Prof. Emery, in volume two. 



11. CAVERNS. 



Although a large part of Iowa is underlaid by limestone 

 formations, there are no caverns in the State that are remark- 

 able as such; yet, in consequence of associated circumstances, 

 some of those that do exist are worthy of mention in this 

 connection. The most noted of these are the lead caves of 

 Dubuque and the ice-cave of Decor ah. The lead-caves are 

 usually mere vertical fissures extending through a great 

 portion of the whole series of strata which constitute the 

 Galena limestone formation. They vary in width from a 

 mere fissure to two or three yards. They are very numerous 

 and are fully described by Prof. Whitney, in his chapter on 

 the Dubuque lead region in the former geological report. 



The Ice-Cave at Decorah is a vertical fissure in the face of 

 the cliff of Trenton limestone that forms a part of the north 

 valley-side of the Upper Iowa river. The fissure is nearly 

 parallel with the face of the cliff, and seems to have been 

 produced by a slight settling away of a portion of its face 

 at the base, causing its slight separation from the solid 

 portion. The fissure or cave is about one hundred feet long 

 in alf its windings, and varies from two to six or eight feet in 

 width. Its height is irregular, as the two walls come together 

 irregularly at the top. 



The ice collects upon its bottom and upon its inner wall 

 near its base, in the form of an encrusting mass. No water 

 was seen flowing or accumulating anywhere; the ice seemed 

 dry and well frozen, and was evidently accumulating at the 

 time of our visit, (June 1st, 1869). The cave was cool and 

 apparently dry, and no strong current was passing through 

 it. The ice is said to be most abundant about midsummer 

 and entirely absent in winter. 



The formation of the ice is probably due to the rapid 

 evaporation of the moisture of the earth and rocks, caused by 



