CONGLOMERATES OF WESTERN INDIANA. 15 



lain by the Lower Carboniferous limestones and the older Paleozoic rocks which 

 outcrop to the east. 



The entire thickness of the formation varies from 20 feet to more than 100 feet. 

 Probably 90 per cent or more of this is a medium coarse-grained sandstone, the 

 remainder being a conglomerate varying locally in the composition and size of its 

 constituents. Occasionally the conglomerate is as much as 20 feet in thickness ; 

 more commonly only a few feet, sometimes but a few inches, and in some localities 

 does not appear at all, being replaced by the sandstone. 



The constituents vary in size from those no larger than wheat grains to some 

 several inches in diameter. Some are hard, crystalline, white or milky quartz; 

 others are of fossiliferous gray chert. 



We should naturally look to a granitic or crystalline area for the quartz material, 

 but the nearest localities are the Atlantic coast on the east, the upper lake region 

 on the north and the Iron Mountain region of Missouri to the southwest, distant 

 200 miles or more in any direction. The transportation of so much coarse gravel 

 such long distances without leaving a perceptible trail leads one to seek elsewhere 

 for a source before going to great extremes to explain how the transporting was 

 done. 



The southwest dip causing the outcrop of the older rocks to the east suggests a 

 careful scanning of these older rocks for the quartzitic material. Previous to my 

 field work I did not regard these Paleozoic rocks as a possible source, but in the 

 field I found great masses of chert conglomerate, sometimes 10 to 20 feet in thick- 

 ness, the fragments being similar, even to the fossil contents, to the chert nodules 

 and layers found in the Lower Carboniferous or Mississippian limestone. So plain 

 is the resemblance that a simple field examination is sufficient to convince any one 

 that such is their origin. 



In a number of places crystallized quartz geodes, entire and rounded fragments 

 of the same, are thickly intermingled with the rounded pebbles of quartz similar 

 in its appearance to that in the geodes. The examination of a few such places is 

 sufficient to raise the question, Why may not all these quartz pebbles have this 

 source? This naturally leads to the inquiry as to whether they were sufficiently 

 abundant to supply the material. 



Persons who have seen only the few r choice museum specimens of geodes would 

 seriously question whether they were ever numerous enough to furnish the mate- 

 rials in the conglomerate, but a few days travel near the area of the Keokuk lime- 

 stone east of the conglomerate would show that the supply is by no means small, 

 and a summer's field work along this area would convince one (or at least did con- 

 vince the writer) that whether such was the source or not, the supply of geodic quartz 

 would be both a possible and probable source of the quartz pebbles in the con- 

 glomerate. If such should prove to be the case it would naturally lead to the more 

 general question whether or not the supply in other localities might not be nearer 

 the conglomerate beds than has been commonly supposed. 



In brief, then, it may be said that field investigation shows that some of the con- 

 glomerate at the base of the Coal Measures in western Indiana has been derived 

 from the segregated chert in the underlying limestone, and that at least some, 

 probably a large part, has been derived from the quartz geodes of the Lower 

 Carboniferous limestone. The question is raised whether all of the coarse mate- 

 rials of the conglomerate may not have thus originated. 



