GEOLOGICAL AND NATURAL HISTORY SURVEY. 49 



direct glacial deposition even in portions of the plain which lie 

 within the limits of the beaches. The clayey matrix of the soft till 

 is scarcely so thickly set with stones as that of the underlying hard 

 till. In both tills many of the stones are glaciated. The rocky 

 constituents grade from bowlders two to three feet or more in 

 diameter down to minute pebbles. These in the upper or later till 

 are made up in large part from the local Upper Silurian rocks, 

 probably less than lo per cent, being from the pre-Cambrian Cana- 

 dian rocks. Devonian rocks from ledges to the north of Chicago 

 aresparingly represented. The clayey matrix is highly calcareous 

 and under the microscope it is found that angular or but slightly 

 rounded grains of limestone constitute a large proportion of the 

 fine material. With the minute limestone fragments, there appear 

 quartz grains, bits of shale and fragments from crystalline rocks of 

 various kinds. Whether the rock constituents of the lower till 

 differ markedly from the upper has not been ascertained. Its sit- 

 uation immediately upon the Niagara limestone would in all prob- 

 ability result in the incorporation of an even larger proportion of 

 this rock than appears in the upper till. 



One of the most conspicuous instances of the occurrence of 

 the lower till within the Chicago area is that brought to light in 

 the excavation of the Chicago Drainage Canal. Immediately east 

 from Summit the canal for a mile or so extends a few feet into a 

 very hard, partially cemented till apparently of early glacial age. 

 Its hardness compared with that of the overlying till is so marked 

 that the contractors who engaged to. excavate this part of the 

 channel were obliged to abandon the steam shovel which had been 

 used in the soft till and resort to blasting. It is probable that this 

 old drift fills depressions in the rock quite extensively in this dis- 

 trict, but as no special attention has been given its identification 

 the instances recorded are not numerous. The well drillers usually 

 recognize the hard till and apply to it the term hardpan, while the 

 soft till is called clay. 



Althouo-h the great body of the drift is till, there are found 

 numerous thin beds of sand or gravel in which water collects in 

 sufficient quantity to supply the wells. There are also small pock- 

 ets of dry sand or gravel occupying but a few cubic feet each. 

 Such pockets were found in the excavation of the main lake tunnel 

 for water supply. They were in some cases completely inclosed by 

 till. 



On the surface of the plain both above and below the level 

 of the upper beach there is quite generally present a clay, varying 



