118 PROCEEDINGS OF WASHINGTON MEETING. 



limits and would abutagainst the underlying strata, [f subsidence were arrested in 

 the interior and continued over the margin, coal beds might be formed in the 

 interior which were not represented over the margin. 



Figure 31 is an ideal representation of what would result with a certain sequence 

 of events of the character suggested. At B is a coal bed, originally horizontal, 

 which extended entirely across a submerged area before subsidence set in again. 

 At 6' is another bed which extended, however, only a short distance before being 

 submerged. At A is a third coal bed winch had a longer period of growth than ( ', 

 but which was also cut off by a sinking of the strata. From the divergence of the 

 lines .1 and />' it is evident that the rate of subsidence was greaterover the interior 

 than at the margin. Before the deposition of the bed B the margin at .1 was ele- 

 vated and the depression in the interior continued, and these opposite movements 

 were kept up during the periods of accumulate >n of tin' strata E and F and of those 

 intervening between these. The next section (figure 32) represents the same group 

 of beds after they have been elevated above, the water, so that the upper beds are 

 elevated some 4(10 feet above the extreme margin. It is, of course, impossible to 

 represent in any such diagram the infinitely complex association and the varied 

 succession of strata which resulted from all the combinations of conditions which 

 probably prevailed during the deposition of the .Missouri ( loal Measures, but, always 

 allowing for the great distortion of thicknesses and of angles of dip and slope, tins 

 diagram will probably suggest all of these. 



The careful study of the above outlined hypothesis and of the last diagram will 

 show that it is calculated to satisfy fully all of the conditions enumerated on page 

 114. Such a study will reveal : 



a. Flow a moderate amount of erosion might suffice to produce the present limita- 

 tions of the upper strata. 



//. Why coal beds are more abundant over the marginal area. 



c. Why the interval between any two strata may be very different at different 



points. 



<I. Why a columnar section, constructed from outcrop measurements of succes- 

 sively exposed strata from margin to topmost layer, will not represent the succes- 

 sion of rocks in such a section as (>, in figure 7. 



e. Why a coal bed may at different points immediately overlie strata which are 

 widely separated from each other in some exposed section, and hence why two 

 separated outcrops of the same coal bed may easily be mistaken for outcrops of 

 two different beds. 



/'. Why the strata cropping out along the margin are not necessarily the lowest 

 beds, even though they dip toward the interior, and why beds encountered at the 

 base by drilling in the interior may be of earlier age than these marginal beds. 



</. That the arenaceous character of the marginal deposits is an essential attrib- 

 ute of their location and not one of their age, and that sandstone, shale, or lime- 

 stone may be prevalent among the upper or lower beds of the Coal Measures 

 according as they were marginal, shallow-water, or marine portions of the deposit. 



Something like a true section of these Foal Measure strata may ultimately be 

 constructed by the present state survey after all the many sections and records 

 obtainable have been studied and correlated. Until then we must proceed with 

 extreme caution, with the anticipation that all the intricacies of deposition which 

 the conditions herein referred to call for may exist and will have to be traced. 



