IOWA ACADEMY OF SC1EN(;ES. 31 



places the alternatiug quartzitic layers are only one or two inches in thick- 

 ness, and are each separated by several inches of loose sand. By selecting 

 the sand grains near the qnartzite and examining them carefully under a 

 microscope, the grains may be found abundantly showing secondary enlarge- 

 ment. In many cases the crystalographic faces are well defined, and the 

 common hexagonal pyramid of typical quartz is found perfectly repi'oduced, 

 each with a sand grain inside. In many instances the sand grain is especially 

 well defined for the reason that red oxide of iron has filled the irregularities 

 in the surface. It appears, then, that in these enlargements there is a more 

 or less rounded irregular grain, thickly coated with iron oxide, and around 

 this has been deposited secondary quartz with crystal faces often well 

 defined. As the secondary enlargement goes on the contiguous grains 

 become closely interlocked, forming the compact vitreous qnartzite which 

 is so well known. 



ORIGIN OF THE PRESENT DRAINAGE SYSTEM OF WARREN 

 COUNTY. 



J. L. TILTON. 



Synopsis: First.- -In Warren county the drift is of uneven depth. As in 

 other drift areas, this unevenness is not dependent entirely on the pre-glacial 

 surface. In the unconformity of the drift on this pre-glacial surface a rela- 

 tion is seen indicating a similarity between the present drainage system 

 and the pre-glacial drainage system. 



Second. — The present river valleys and larger ravines are larger than 

 present streams require. They fit into the pre-glaciai valleys. 



Third. — In the smaller ravines only do we find erosion without regard to 

 the pre-glacial configuration of the county. 



In connection with field geology work in the northern part of Warren 

 county and the adjacent townships of Madison county, a question of con- 

 stantly increasing interest to me has been this: To what is the present 

 drainage system of the county due? I will endeavor to make clear an 

 answer to this query, without too much detail, leaving other questions to l)e 

 presented at some future time. 



It is generally understood that the drift is laid in irregular deposits, here 

 thick on the hill tops, there thick in the valleys. Are we to expect, then, that 

 the present drainage system has been marked out since the "Ice Age," with lit- 

 tle regard for the previously existing systems? It is true, that in the county 

 referred to there is no regularity in the depth of the drift deposit. At times 

 the drift rests on sandstone, at times on limestone, at times on shale. 



Two-thirds way from lujianola to Spring Hill is a valley; its sides with 

 equal pitch. The road down the east side shows Carboniferous outcrops 



