A. Winslow — Cambrian in Missouri. 225 



State. In this period the writer has carefully examined 

 numerous outcrops and exposures and has, further, had the 

 benefit of the results of a large amount of diamond drilling 

 and shafting which have been done here, notably in the vicinity 

 of Bonne Terre, Flat River, Farmington, Doe Run, Iron 

 Mountain, Mine La Motte and Fredericktown. The results of 

 this work all go to show that in St. Francois and in the north- 

 ern part of Madison counties the sedimentary rocks between 

 and beyond the granite and porphyry hills may be divided into 

 the following three parts from the top downwards. 



Observed thicknesses. 

 Feet. 



1. Limestone, magnesian, crystalline; immediately under- 



lies most of the valleys and constitutes the 

 bulk of the non-Archaean hills. In thin and 

 massive beds; includes some shale, espe- 

 cially in the lower parts ; arenaceous layers 

 encountered at places, but are of subordi- 

 nate importance and not persistent. This 

 is the lead-bearing horizon of southeastern 

 Missouri 1 to 500 



2. Sandstone, of white, gray and reddish colors ; gene- 



rally composed of pure quartz grains with 

 secondary enlargements liberally devel- 

 oped ; sometimes thinly bedded, even shaly, 

 elsewhere massive, but generally friable 



and difficult to drill through 1 to 100 



S. Conglomerate, composed of granite or porphyry bowl- 

 ders with a limestone, grit or clayey matrix, 1 to 50 

 Granite or porphyry floor. 



This section is, of course, not represented in full every- 

 where. The best succession is perhaps found in the center 

 or the broader valleys, between the Archaean hills. As one 

 approaches the sides of the valleys any or all of the beds may 

 taper out. The detrital conglomerate would, naturally, not 

 be found resting against a steep granite wall, but the bowl- 

 ders would have slid or rolled to lower levels ; the lime- 

 stone, being often at higher level than the sandstone, may 

 extend beyond the latter up a concealed hill slope of gran- 

 ite or porphyry and, thus, be often directly in contact with 

 either of these latter rocks, or with the conglomerate derived 

 from them. On the other hand, a thickening of the sandstone 

 towards its source, an inclination of the floor or a slight dip, 

 aided by erosion, may bring the sandstone to view at a higher 

 elevation than the geologically higher limestone. This is the 

 case in the vicinity of Doe Run ; about three miles southeast of 

 that place the sandstone, which underlies the limestone con- 



