UPPEE SILURIAN SYSTEM. 183 



This form necessarily changes the trend of the succeeding 

 formations several degrees nearer to the northward and 

 southward than that of the preceding formations is. 



The area occupied by the Niagara limestone is nearly a 

 hundred and sixty miles long from northward to southward, 

 and between forty and fifty miles wide in its widest part. At 

 its narrowest part, which is near its northern limit in Iowa, it 

 is not more than four or five miles wide. The narrowing of 

 the area here is due to two causes; namely, the thinning out 

 of the formation in its northward extension, and the increased 

 westerly dip of the same, together with that of its associated 

 formations in that part of the State. That increase of dip is 

 well shown in the section from McGregor to the mouth of 

 Broken Kettle creek. The rapidly increasing width of the area 

 which the formation has to the southward, is not wholly due 

 to its increasing thickness in that direction, but is due also to 

 disturbance of its strata and change of dip. In consequence 

 of this, and also of the irregularity and diversity of its strati- 

 fication, no satisfactory conclusions have yet been arrived at 

 concerning its thickness. It is estimated at two hundred and 

 seventy-five feet, but its maximum thickness in Iowa is 

 probably much more. 



A line of disturbance of the strata of this formation, passes 

 into Iowa in a direction almost corresponding with the direc- 

 tion of its trend; crossing the Mississippi river at LeClaire, 

 and being last recognized in the valley of the Wapsipinicon 

 about three miles west of Anamosa. This disturbance appa- 

 rently partakes more of the nature of an abrupt fold than of 

 a true fault, leaving the general dip of the strata on each side 

 of its axis seemingly unchanged from what it would have 

 been if the disturbance along that axis had not taken place.* 

 This disturbance seems confined to a very narrow area, for 

 the strata within less than a mile on each side of both the 

 localities named, appear entirely undisturbed, although the 



*This is the only known instance in Iowa of a fold having the general direction of 

 any of the streams. Other slight folds exist, as will be shown in the general sections 

 accompanying this report, but they do not coincide with the courses of the streams. 



