246 THE PHYSICAL MANIFE STATIONS OF WEATHERING 



however, as has been observed, soluble in the carbonated water 

 of rainfalls, and, in time, may undergo complete removal, 

 leaving but the insoluble impurities behind. This is, indeed, 

 the almost universal history of limestone soils. They are not 

 infrequently so siliceous or ferruginous as to be quite barren 

 and of a nature to be benefited by the application of lime as a 

 manure. 



Throughout the areas occupied by the Trenton limestones, in 

 Maryland, nearly every farm has, in years past, had its quarry 

 and lime-kiln where the stone was fitted for supplying lime 

 once more to soils from which it had been so thoroughly leached 

 as to render them lean and poor. It is to this solvent action 

 that is due the formation of the multitudinous caverns, large 

 and small, of the limestone regions. Even where caverns are 

 not apparent, the corrosive action is evident to the practised 

 eye. In the quarry regions of Tennessee surface blocks of 

 limestone are often grooved to a depth of an inch or more 

 with wonderful sharpness, simply from the water of rainfalls 

 with its acids absorbed from the atmosphere and surface soils, 

 while in the quarry bed the stone is found no longer in con- 

 tinuous layers, but in disconnected boulder-like masses. (Fig. 

 2, PL 23, and Fig. 3, pi. 14.)^ In such cases casual examinations 

 give very little clew to the rapidity of the destruction going 

 steadily on, since all is removed in solution excepting the com- 

 paratively small amount of insoluble matter (usually clay or 

 silica) existing as an impurity. 



(9) Incidental Surface Contours. — In limestone regions the 

 solvent action of water has frequently gone on so extensively 

 as to leave its imprint upon the topographic features of the 

 landscape. Narrow, symmetrical valleys, due wholly to solution, 

 have been described^ in what is known as the Boone chert region 

 of northern Arkansas. Such have steep slopes and are of great 

 length in proportion to their width. In many a limestone area 

 the drainage is no longer wholly superficial, but by subterranean 

 streams sinking entirely into the ground to reappear again at 

 lower levels, it may be miles away, having traversed the inter- 

 vening distance in some of the numerous passages (fissures en- 

 larged by solution) with which the rocks abound. Entire land- 



* These are eYidently identified witli the so-ealled ''karren'' forms of 

 German writers. See Globus, Vol. 70, No. 7, August, 1896. 



* By Messrs. Marbut and Perdue, Jour, of Geology, 1901, p. 47. 



