250 GENEEAL GEOLOGY. 



Peotozoaists are represented in the greatest abundance by 

 Fusulina cylindrical some layers of limestone being almost 

 entirely composed of tlieir small fusiform shells. Another 

 form, probably Protozoan, and belonging to the genus 

 ArripMstegina has also been found in these strata; but the 

 details of its structure have not yet been fully made out. 



4. GENERAL OBSERVATIONS UPON THE CARBONIFER- 

 OUS ROCKS OF IOWA. 



It will be seen from the preceding descriptions of the 

 formations which compose the two groups of the Carbon- 

 iferous System in Iowa, that the aggregate thickness of the 

 four formations of the Sub-carboniferous group is not far 

 from five hundred and fifty feet. The thickness of the three 

 formations of the Coal-measure group is by estimate and 

 actual measurement about six hundred feet, not allowing in 

 either case for thickening of the strata to the southwestward, 

 which, doubtless, takes place to some extent at least. Accor- 

 dingly, the entire thickness of the Carboniferous strata in 

 Iowa is put down at about eleven hundred and fifty feet. 



With the exception of rare, and usually unsatisfactory 

 results of artesian borings, it is of course only by measure- 

 ments made upon the outcrops of any formation that we are 

 able to ascertain its thickness. We are consequently not able 

 to say with precision where and how much a formation may 

 thicken, or where it may thin out after it has passed from 

 sight and reach beneath other formations. It is also often 

 difficult to ascertain whether or not a certain formation 

 increases or decreases in thickness in any given direction, 

 especially if it has a dip so slight that it corresponds nearly 

 with the general drainage-slope of the region it occupies, as 

 for example the Upper coal-measures of a large part of 

 southwestern Iowa do. As a rule, however, it may be inferred 

 that the thickening, if any occurs, will be found to increase 

 in the direction of the centre from the border of the area 

 occupied by the formation, or in any direction within it at 

 right angles with its border. 



