GEOLOGICAL RESULTS OP INDIANA COAL SURVEY 9 



to the north, while several hundred feet of strata and many coal beds were being 

 laid down in the southern and southwestern part of the field before the coal-form- 

 ing conditions had reached the present northern limits of the coal area. This slow 

 overlapping from the southwest toward the east and north was one of the most 

 interesting discoveries made by the survey. At the extreme north coal vi is found 

 resting unconformably on the basal sandstone, while at the south this coal outcrops 

 20 or 30 miles west of the sandstone, from which it is separated by over a dozen 

 coal beds and their intervening strata. 



It would be out of place here to go into the detailed history of the laying down 

 of the main measures, though a large number of interesting features were worked 

 out. Nonconformities occur at numerous places in the series of events, usually 

 with erosion levels with a difference of less than 20 feet. In one case, however, 

 the streams eroded their channels to depths of up to nearly 200 feet, cutting out 

 the coals and other strata. These channels are abundant over the north part of 

 the field, and are well exposed in Parke, Fountain, and Vermillion counties, due 

 to the extensive quarrying of the sandstone with which they are filled. These 

 channels may correspond to a nonconformity found further south between coals via 

 and vi&, or to one existing between the Coal Measures and the massive Merom 

 sandstone which overlies them. 



Due to the overlap described, the thickness of the Indiana Coal Measures is varia- 

 ble, ranging from about 1,000 feet in the southwest to only a few hundred feet at 

 the north. 



The Coal Beds 



In number, as high as 17 beds have been found in a single drilling, with a total 

 thickness of over 32 feet. At least 20 coal horizons outcrop, and, counting the 

 overlapped beds that do not outcrop, it is possible many other horizons exist. 

 Exact information about the lower outcropping coal beds might increase the num- 

 ber given, as the persistence of the lower beds is assumed and not real. 



In extent the coal horizons vary greatly. Some of the upper hori zons are thought 

 to have been traced the whole length of the coal field. Thus what we have called 

 coal vii would appear to have been a practically continuous bed from the Ohio 

 river to where its outcrop crosses the Illinois line, in Vermillion county. In the 

 same way we have traced the horizon of coal vi continuously, though the coal is 

 not continuous. To the south it runs out, to the north it becomes pockety, but 

 between are two basins of several hundred or a thousand square miles each, where 

 the coal is thick, persistent, and extremely regular in its details, clay or pyrite 

 bands from a fraction of an inch to two inches thick persisting over the whole of 

 the basins ; so of many of the other of the upper coals, and also the accompanying 

 beds, especially the limestones. 



Going down to the horizons of coals ii, in, and iv, the coals are found to occur 

 in small basins, often of only a few acres, the coal running from 3 to 5 feet thick 

 in the center of the basin and often running down to as many inches or nothing 

 over the elevated divides between the basins. Yet even in such cases it is often 

 possible to trace partings and other minor stratigraphic details of the coal from one 

 basin to another over areas of several hundred square miles. The basin structure 

 in most of these cases would seem to be due to the irregularities made by the ero- 

 sion of the subcoal surfaces. These coals tend to be " block coals," having a remark- 

 ably perfect system of joint planes, besides usually being non- caking. 



In thickness the coals range up to 10 feet. Several of the upper beds will main- 



II— Bull. Geol. Soc. Am., Vol. 11, 1899 



