BAIN.] DESCRIPTION OF SEDIMENTARY FORMATIONS. 27 



QUATERNARY l>EPOSITS. 

 LOESS LOAM. 



Throughout the two counties there is commonly a surface material 

 of light-buff, pebbleless clay or loam. This is spread as a thin mantle 

 over the underlying rocks, and over the flat uplands, in particular, is 

 widespread. On the slopes it is cut away or so mixed with residual 

 material as to be indistinguishable. In places the sand contributed 

 to it by the decomposition of the local rocks is very abundant. In 

 other places, however, it exhibits all the characteristics of upland 

 loess of the glaciated area farther north, and it is doubtless a phase 

 of one of the older loess sheets ^^'hich have been there discriminated. 



ALLUVIUM. 



Along the Ohio and many of its tributaries are numerous broad, 

 flat bottom lands underlain by alluvium. No attempt has been made 

 to discriminate thejn on the accompanying maps, nor has any study 

 been made of the interesting and complex river history which they in 

 part record. 



DESCRIPTION OF IGNEOUS ROCKS. 

 GENERAL STATEMENT. 



The discovery of intrusive igneous rocks in Illinois was very recent. 

 In 1889 Mr. Ulrich, while working for the Kentucky Geological Sur- 

 vey, discovered the Flannery dike in Crittenden County, Ky., and in 

 1892 Mr. J. S. Diller described the rock. From time to time other 

 dikes were found in that State, until now a considerable number are 

 known. In 1902 Mr. W. S. Tangier Smith located the Mix dike above 

 Golconda, that being the first found in Illinois. In the course of the 

 work in 1903 a number of additional dikes were found, and there are 

 doubtless others which as yet remain undiscovered. Near Hicks Mr. 

 Crider found consideral)le mica in one of the streams, and Mr. F. 

 Julius Fobs has furnished a specimen of a mica-bearing rock picked 

 up nearby. So far, however, it has proved impossible to find material 

 in place. 



The dikes so far found are indicated on the accompanying maps. 

 In no case has a dike been traced any considerable distance, and with 

 two exceptions they are found only in the bluffs of the Ohio and for 

 a short distance inland. The dike in Downey's bluff at Rosiclare 

 sends off a thin sheet which is intruded into beds of the Ohara member 

 of the Ste. Genevieve. The dike at Orr's landing has a thin stringer 

 running off' into the St. Louis limestone. With these exceptions the 

 igneous rock so far as known occurs only in normal, vertical dikes. 



