240 APPENDIX. 



rolling hills of the prairie, like ruined castles, moss-grown 

 under the hand of time. 



" Sometimes they present, even when more closely in- 

 spected, a curious resemblance to turrets, and bastions, and 

 battlements, and even to the loopholes and embrasures of a 

 regular fortification. Sometimes single blocks are seen jutting 

 forth, not unlike dormant windows rising through the turf clad 

 roof of an old cottage ; and again, at times, especially along 

 the descending spurs of the hills, isolated masses emerge in 

 a thousand fanciful shapes, in which the imagination readily 

 recognizes the appearance of giants, sphinxes, lions, and 

 innumerable fantastic resemblances. 



" The appearance of this rock is further modified by the 

 peculiar manner in which it weathers. Numerous masses of 

 chert (a variety of flint), and also many siliceous fossils, are 

 interspersed through its mass ; and these, becoming gradually 

 loosened by the action of air and water, drop out, and leave 

 cavities of various shapes and sizes. Thus the rock is fre- 

 quently found riddled with irregular holes, from a few inches 

 to a foot in diameter, giving its surface a rugged and almost 

 bone-like appearance. Frequently this variety in the compo- 

 sition of the rock gives occasion to an undermining process 

 on the lower surface of a cliiF, which gradually proceeds, 

 until, perhaps, a towering and tottering column remains, sup- 

 ported on a contracted base, which threatens every moment 

 to give way and precipitate the poised mass into the valley 

 beneath. 



" The cliff limestone of Iowa is, strictly speaking, a mag- 

 nesian limestone, containing (by careful analysis of four 

 separate specimens from diff'erent localities) from thirty-five 

 to forty per cent, of carbonate of magnesia. 



" It contains, on the average, from eighteen to twenty per 

 cent of pure magnesia ; and by mere solution in sulphuric 



