432 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



The ruptures are long and somewhat curved. In the other set, 

 which is confined to the western portion of the state, the value of 

 the movement figures is not nearly so great as in the case of the 

 other ; yet it is still quite notable. The space between faults repre- 

 sents a distance of about twenty-five miles. This figure suggests 

 the spacing value of the entire system. Plotting upon the map of 

 the state other lines to mark possible positions of other faults 

 we find abundant indications of the presence of such features. 



In partial explanation of this phenomenon we get an inkling from 

 a neighboring source. It is a well known fact, established through 

 extensive experience in mining operations, that when the interval 

 between two parallel faults is determined other faults are expected 

 CO exist at like intervals. This circumstance is directly traceable to 

 the nature of the tortional strains which rock-masses undergo. 

 Whether or not such a high spacing value as twenty-five miles is act- 

 ually possible remains to be determined theoretically. The problem is 

 readily susceptible of mathematical demonstration, as in the cases 

 of fault-systems of much closer pattern. It would be exceedingly 

 instructive to apply the principles involved to the Iowa situation. 



ANTIQUITY OF IOWA'S OLDEST ROCKS 



Of late years the stratigraphic level of our lowest rocks is pushed 

 back immeasurably. Now they rank with the oldest of any of which 

 we have knowledge on the face of the globe. Terranes older than 

 those of the Paleozoic age occupy in Iowa a very small surface area. 

 Attention which is bestowed upon them is about commensurate 

 with their relative surface extent. Heretofore, one finds that little 

 attempt has been made to determine their broader stratigraphic 

 relationships, their real position in the general geologic column, 

 their possible subdivision, or their role in the geotectonics of the 

 region. It seems all sufficient merely to note the existence of these 

 rocks in the extreme northwestern corner of the state. Yet these 

 very rocks now appear to have a history longer, more complicated, 

 and more vicissitudinous than that of any other terrane represented 

 within our borders. 



For the first time we recently learn that some of these pre- 

 Cambrian rocks are very much younger than was once thought to 

 be the case ; and that others are very nnich older. For the first 

 time, also, we are now able to compare them with a standard 

 section of the most ancient sediments known, that very complete 

 and satisfactory classificatory scheme of the Lake Superior region 



