210 ILLINOIS STATE ACADEMY OF SCIENCE 



black shale and that there have been at least two separate 

 generations of sinks in this one area. In addition to these 

 two types of shale, there are rare occurrences of a granu- 

 lar, fine or coarse grained, calcareous sandstone that is 

 made up of grains of quartz and of the surrounding lime- 

 stone. In several cavities there are deranged masses of 

 limestone that are collapsed portions of the walls, and 

 around these masses the shale is practically undisturbed 

 from its horizontal position, showing that the collapse 

 occurred contemporaneous with the deposition of the 

 shale. Similarly, where there are irregularities of the 

 walls or recessive channels, they are filled by shale that 

 is stratigraphically continuous with that in the main pit. 



Some idea of the extent of the area which exhibits 

 this phenomenon may be gained from the following facts. 

 The Lehigh Stone Company abandoned an old quarry two 

 miles west of the present one because of the prevalence 

 of "clay pockets". While testing prospective areas on 

 which to locate the present plant, they found similar 

 "clay pockets" abundant over a wide expanse of this 

 region. In an old quarry on the west side of the Kanka- 

 kee Eiver at Kankakee it is reported that there were 

 found "soapstone pockets," which are doubtless the 

 same as the usual "clay pockets", containing good shale. 

 Several of the farm wells in the intervening territory 

 have been drilled through "mud" for many feet instead 

 of the usual rock, and this fact, considered in the light 

 of the chance location of well sites, indicates that the 

 shale-filled cavities must be numerous, to say the least. 



In the eastern part of Bradley, two miles north of 

 Kankakee, are two small circular swamps overgrown 

 with willows, which probably mark the location of large 

 shale-filled sinks like the others. Dr. D. J. Fisher of the 

 University of Chicago reports that in the quarries about 

 Joliet are similar cavities filled with green clay or shale, 

 but there the evidence was insufficient to warrant any 

 positive statement as to their origin or age. Smaller 

 cavities in the Niagaran limestone, filled with Devonian 

 shale or clay, have been reported from a quarry near 

 Elmhurst 1 and from McCook, near Summit 2 . 



1 Weller, Stuart : A Peculiar Devonian Deposit in Northeastern Illinois ; 

 Journal of Geol., v. VII, No. 5, 483-488, July-August, 1899. 



- Personal communication from G. W. Hawley, State Geological Survey. 



