108 PHYSICAL GEOGPAPHY. 



is doubtless due to the slight cementation of the minute 

 grains of silica which constitutes so large a part of it. This 

 is effected by the partial solution of its carbonate of lime by 

 the carbonated waters which fall upon and percolate it as 

 before mentioned, and the reprecipitation of the same in 

 those parts nearest the exposed surfaces, whether artificial 

 or natural. The cementing material thus formed, is never 

 sufficient in amount to interfere with its porosity, for this is 

 another of its peculiarities. The cause of its being so per- 

 vious to water is no doubt due to the distinct granular form 

 of its silex although so minute, and the fact that the minute 

 grains are not compacted and rendered coherent by impalpa- 

 ble clayey matter. Thus water passes through it just as 

 effectually as though ordinary sand, but not so quickly. 



That the deposit is very pervious to water may be readily 

 seen by observing the line of springs that issue at its base 

 near the foot of the bluffs, and nowhere above that line. 

 That it is so pervious, also appears from the fact that wells 

 dug in it, so far as has yet been ascertained, have never 

 afforded a supply of water until the base, or very near the 

 base of the deposit was reached. Often the whole thickness 

 of the deposit must be passed through in digging the well, 

 and the water when found, is obtained in the sandy or 

 gravelly drift which everywhere underlies the Bluff Deposit. 



It will thus be seen that this strange deposit, although it is 

 compact, has the property of being very porous, so that the 

 water which falls upon its surface never collects there in 

 ponds, nor does it accumulate within its mass as it is known 

 to do upon the surface of and within the drift and the 

 stratified formations. Consequently, there is great risk that 

 wells dug in its deepest parts must necessarily be dug to an 

 impracticable depth before reaching water, for a well two 

 hundred feet deep would hardly be practically useful. 

 Where the deposit is thinner, however, it may be entirely 

 pierced, and water obtained at little if any more than the 

 ordinary depth required in the drift deposit alone, which 

 forms the surface of the greater part of the State. Although 



