THE DRIFT-DEPOSITS OF THE NORTHWEST. 209 



one of them. For example, in Ohio (Northwestern Ohio) it is very- 

 close and clayey, the upper part being free from bowlders and stones. 

 In Minnesota (Central Minnesota), the stones and bowlders are more 

 generally disseminated throughout the whole, and it shows much more 

 sand. Hence, in the latter State it will allow the slow passage of water 

 through it a fact exceedingly fortunate for the agricultural capacity 

 of the soils that are based on it. 



Perhaps there should be added to this description of the drifts an- 

 other member, which may be denominated No. 4. 



No. 4. This, however, is so inconstant and so often runs into the 

 last, that it is hardly worthy of special designation. When present, 

 it lies below No. 3, and immediately over the rock. It consists of 

 gravel and bowlders, more or less mingled with clay. It is the great 

 water-reservoir that supplies deep wells, and through it there is a con- 

 stant slow drainage into deep valleys and excavations. It gives rise 

 to springs at the base of the river-bluffs, and to artesian wells, when 

 the confining stratum of hard-pan above is penetrated in regions of 

 favorable slope. In Northwestern Ohio is a most wonderful series of 

 artesian wells that depend entirely upon this fortunate combination 

 of circumstances. This member of the drift sometimes consists largely 

 of assorted materials, as sand and gravel. This is indicated by the 

 issuing of considerable quantities of such sand from artesian wells 

 that penetrate it, when the force of the current of water is sufficient 

 to bring it to the surface. Instances have occurred of the collapse of 

 the overlying clay and hard-pan, on the removal in this way of large 

 quantities of sand. Along the upper side of this member, the mate- 

 rials are apt to be firmly cemented by iron and carbonate of lime, into 

 a very firm, even rocky crust. Such cemented gravel and sand is sel- 

 dom over two feet thick, yet well-borers, on reaching it, often mistake 

 it for the bed-rock. They pass through it, and usually find a copious 

 flow of excellent water. The announcement is then promulgated that 

 that well was drilled through the rock . Hence it is a popular error 

 that, in order to obtain water of the purest quality, it is necessary to 

 sink a well "through the rock." This member, though, is not always 

 found in penetrating the drift to the rock. It may be here stated, 

 also, that, when No. 4 is present, the bed-rock does not show so plainly 

 the marks of glaciation ; but, when No. 3 extends to the rock, the sur- 

 face of the bed-rock is almost invariably marked with the well-known 

 crlacier-etchings. 



Having taken this survey of the component parts of the drift, it 

 will be well to bring them into a general view, as illustrated by the 

 preceding diagram (Fig. 3), in which the figures represent the same 

 members as in the previous illustrations. It shows a general section 

 of the drift, from Toledo to Shelby County, Ohio, and is based on ex- 

 tended observations, the results of which are to appear in the forth- 

 coming reports of the " Geological Survey of Ohio." The six ridges 



VOL. III. 14 



