LOCAL GEOLOGY OF DAVENPORT BAEEIS. 265 



from some member of the coal measures. It could hardly have belonged 

 to a later formation. 



In other cavities occurs a tenacious blue or greenish clay, having some- 

 what the appearance of a hre-clay, and to some extent used for that 

 purpose, how successfully I know not. It possesses a uniform consis- 

 tency, varying little in character or color with the depth to which it has 

 been exposed. It is so difficult of removal that the quarrymen leave 

 large masses of it in place after removing the surrounding rock, so that 

 in the quarries they still stand up in pyramidal forms, or in case tlie 

 quarries are overflowed with water, they constitute the islands appearing 

 above the surface. 



In passing I would call your attention to what appears to have been a 

 regular subterranean water course. In Cook's quarry is a mass of clay, 

 twenty or thirty yards in length, three feet broad, and in depth extending 

 down nearly if not quite to the '' flint rock" before referred to. No work- 

 man could lay up a series of stone layers presenting a better facing than 

 that exhibited by these walls. The curves are frequent and gradual. The 

 filling of clay is so difficult to deal with that the workmen blast down to 

 " flint rock," then cross over and begin their work on the other side. 

 While so firm is the clay, that after the rock has been removed from its 

 sides, it maintains its erect position, and for days in pleasant weather 

 retains all the impressions made by the abutting rocks. This blue clay, 

 w^hether confined in cavity or extended in this ancient water course, is of 

 the same character as that described by Prof. Hall as occurring in the 

 quarries between Moline and Rock Island, and which he regards as hav- 

 ing originated in the coal measures, finding in it in that locality a Euom- 

 phalus, distinct from any in the surrounding rock, and very similar to a 

 carboniferous form. This, then, is no doubt the origin of the blue clay. 



But we come to a fact new to science, as first developed in these quar- 

 ries. Side by side with these reservoirs of sand and clay from the coal 

 measures we have immense cavities, filled with the soft shale of the 

 Hamilton*. The bedding is generally irregular, no doubt in its lower part 

 conforming to the irregularities of the rock in which it has been depos- 

 ited, in the upper partaking somewhat of the irregularities of the roof, 

 yet everywhere preserving traces of the layers. 



The gentleness of the deposit may be inferred from the fact that this 

 shale is ci'owded to repletion with immense numbers of the smallest 

 shells of the Hamilton, in the most complete possible state of preserva- 

 tion. AVithin the space of a few feet, after every rain, hundreds of the 

 small Chonetes, with even their spines preserved, are washed out. 



*Some of these cavities are distinguished by huge masses of carbonate of lime, most gener- 

 ally presenting the appearance of a crowded, confused acicular crystallization. The form of 

 the mineral, in some instances, sugs^ests its having been originally suspended from the roof of 

 the cavity, and then by some meaus being detached and precipitated into the clay. Other spec- 

 imens have every appearance of having been formed where found, as they partake of all the 

 irregularities in the deposits of the clay by which surrounded, while others still have been 

 rounded by the action of water until they are worn quite smooth. Fossils are scarce in this 

 class of cavities. 



