36 



IOWA ACADEMY OF SCIENCES. 



PRISMATIC SANDSTONE FROM MISSOURI. 



BY ERASMUS IIAWORTH. 



(Published by consent of the State Geologist of the Geological Survey of Missouri.) 



On the right bank of the St. Francois River, in S. .31; T. 33, N.; R. 6. E., about 

 200 yards southwest of the St. Louis Granite Company's cfuarry, near Knob' Lick, 

 Madison county, Mo., is a little sandstone ridge, trending northwest and south- 

 east, nearlv 200 yards long, 10 yards wide, and not more than 8 to 10 feet high 

 above the nearly level ground on either side. The country rock here is the Cam- 

 brian sandstone, which overlies the granite, as is beautifully illustrated at the 

 quarry near by. This little ridge is interesting on account of the peculiar form of 



the sandstone composmg it. 



In places where the soil has been somewhat worn 

 away, instead of revealing flat layers of sandstone, as 

 can be found near by in any direction, the surface is 

 covered with fragments of sandstone of a prismatic 

 form, resembling in shape the basaltic columns so 

 well known in different parts of the world. In size 

 the prisms range from about three-fourths of an inch 

 to one and a half inches in diameter, and from three 

 to eight inches in length. They are not uniform in 

 geometrical outline, some having four sides, some 

 five, and a few six. Quite often two and occasion- 

 ally three prisms adhere together, side by side, but 

 generally so loosely that they can easily be broken 

 apart. In such cases the boundary between them is 

 usually a single plane, but sometimes two new planes 

 are exposed by the lireaking, forming a re-entrant 

 angle on one prism. Fig. 1 fairly represents a com- 

 bination of two of these prisms. 

 The nature of the rock was studied cfuite carefully, both macroscopically and 

 microscopically, and it was found to be nothing but an ordinary, somewhat irregu- 

 larly indurated, fine-grained sandstone. The grains of quartz are waterworn, as is 

 usual. The induration is produced by the interstitial spaces being more or less 

 filled with silica, but the thin sections examined showed no instance of secondary 

 growth of the quartz crystals. 



The existence of the ridge is probably due to the induration of the sandstone. 

 Why this limited area should be thus indurated, and the surrounding country 

 should not be, there seemed to be no obtainable evidence. However, this of itself 



