ABSTRACTS AND DISCUSSIONS OF PAPERS 123 



neatli by a disconformity of considerable magnitude. It possesses a character- 

 istic basal sandstone. As at present restricted, therefore, the Saint Louis 

 represents only about one-half of the strata originally referred to it, or about 

 50 to 60 feet of limestone. 



In southeastern Iowa the formation, as revised, is divisible into two well 

 marked portions, which in their typical development are of about equal thick- 

 ness. These are everywhere separated from each other by a slight discon- 

 formity. The lowermost of these is usually dolomitic, either wholly or in 

 part, and the lower portion of the upper one is sometimes so. To the north- 

 westward the upper division soon disappears, but the lower one continues to 

 the north-central part of the State, overlapping older and older formations, 

 until at Humboldt it is found resting on beds of Kinderhook age. 



Where undolomitized, both divisions of the Saint Louis consist typically of 

 line-grained, compact, brittle, gray limestone, which breaks with a subeon- 

 choidal fracture. Beds of granular limestones a few feet in thickness, how- 

 ever, appear at some horizons, and the lower portion of the lower division is 

 often shaly or arenaceous, or both. The dolomitic portions, on the other hand, 

 are buff in color, somewhat coarser in grain, and tough. These occur both as 

 interbedded layers in the limestone and as lateral gradations or facies of lime- 

 stone beds of considerable thickness. That the formation was deposited in 

 shallow seas is clearly indicated by several features hereinafter noted. 



The brecciation effects in the formation may be grouped into three main 

 types. First, the disturbed portion may assume the form of a small mound or 

 reef of limestone blocks, usually in a calcareous or sandy matrix, with undis- 

 turbed layers lapping up on its flanks and filling in its depressions. These 

 may appear at any horizon in the formation. Second, it may appear along 

 minor faults in the Lower Saint Louis, or be confined largely to one bed of 

 this member, due to differential movements. The latter is accompanied occa- 

 sionally by the tonguelike extensions of the material forced down into the 

 beds below, especially when the latter are softer. Third, it may embrace the 

 major part, if not the entire formation, over an area of variable but usually 

 limited extent. In this type the disturbance has been produced by mashing 

 on a large scale, and the brecciation is often associated with folds and over- 

 thrust faults of small magnitude. The Pella beds are also involved to a slight 

 extent in this disturbance, but in no instances have the underlying formations 

 been found to exhibit signs of this type of deformation. 



Regarding the origin of the brecciation, at least three periods of disturbance 

 are believed to have operated in its production. The mounds or reef-like 

 masses of the first type are believed to have been formed during the first 

 period under conditions of violent wave action, possibly induced by local 

 shallowing of the sea during deposition. The presence of local discontinuities 

 in the formation and of cross-bedded lime sands supposedly formed by the 

 grinding up of layers already deposited is in favor of this view. Other fea- 

 tures which suggest wave action at the time of deposition are contemporaneous 

 erosion phenomena, wave-marks, and cross-bedding. 



The brecciation of the second type was formed during the second period of 

 deformation. The importance of this can not be definitely evaluated, since its 

 effects are often overshadowed by the disturbance of the third type. But that 

 this disturbance is distinct from the third is indicated by the fact that 



