:28 IOWA ACADEMY OF SCIENCES. 



ileast double this estimate; and at a still later date the vertical extent of the other 

 .two formations has been found to differ very much from the limit usually assigned: 

 the Middle Coal Measures being considerably thinner than was supposed, and the 

 Xower Coal Measures very much thicker. 



From an economic standpoint, the coal of the region forms by far the most 

 important deposit. The seams vary from a lew inches to seven or even eight feet 

 in thickness; the average of the veins at present worked being between four and 

 .five feet. These are disposed, not in two or three continuous layers over the entire 

 area, but in numerous lenticular masses from a few hundred yards to several miles 

 in diameter. A single horizon may thus contain several of these lens-shaped beds 

 of greater or less extent. Along the line of the general section the coal-bearing 

 horizons have been found to number more than a score; and the extension of the 

 investigations beyond the limits of the particular area here considered has very 

 greatly increased this figure. Recognizing this fact, the aggregate amount of 

 coal is far in excess of what has been supposed hitherto. The peculiarities of its 

 disposition and the consequent popular misunderstanding concerning the actual 

 ■extent and distribution of the coal beds has led to a large but useless expenditure 

 •of capital. This phase of the question will receive turther expansion in another 

 fplace. 



Summing up the more salient features in the present preliminary consideration 

 •of the Coal Measures of central Iowa, it may be said that: 



1. The Lower Coal Measures are very much thicker than has been hitherto 

 supposed. 



2. The so-called Middle Coal Measures are are not so extensive, vertically, as 

 ■was once supposed; and the designation as a formation name is of very doubtful 

 •utility, at least in so far as Iowa is concerned. 



3. The recognition of the very subordinate importance of the " Middle" member 

 ■suggests that the Coal Measuies in Iowa may more properly be regarded as form- 

 ing two, instead of three, divisions.* 



4. The unconformity of the Lower Coal Measures of Iowa upon limestones of 

 the Lower Carboniterous is much more pronounced than heretofore suspected. The 

 confirmation of this statement is found in excavations recently made at Elk Cliff, at 

 Harvey, at Fairfield, in Jefferson county, and elsewhere. 



5. The striking unconformities in the Lower Coal Measures have never been so 

 apparent as at present. Ihe most remarkable instance of this sort is the case of 

 the Redrock sandstone. The va-st sand bed had manifestly been consohdated and 

 elevated above the surface of the sea for a considerable distance; then it was sub- 

 jected to long-continued denundation, as is shown in the deep gorges and ravines 

 which are still preserved in the hard sandstone. So widespread and intense was 

 the action of the erosive agencies that the great sandstone, more than one hundred 

 and fifty feet in thickness, was largely removed; and at the present day only a few 

 isolated outliers tell of its former great extent. When regional submergence again 

 set in, the old gorges and shore depressions were occupied by coal swamps. 



6. The earliest formed coal seams are far more extensive, both geographically 

 and vertically, than the later ones. On the whole, the coal of Iowa may be regarded 

 as distributed in innumerable lenticular basins, sometimes several miles in diam- 

 eter and six or seven feet in thickness centrally, sometimes only a few hundred 

 yards in extent. These occur at many different horizons and interlock with one 

 another, so that a boring may pass through a score or more coal horizons without 

 meeting more than one or two veins of sufficient thickness tor profitable working. 



