96 PHYSICAL GEOGRAPHY. 



streams are not only coincident but also that they were both 

 determined by a common cause. It is evident, however, that 

 we cannot rely upon the direction of glacial scratches at any 

 one particular point to indicate the general direction of former 

 glacial currents, because these may have changed a few 

 degrees within the distance of as many miles, even in 

 the flattest country, so that the most reliable indications 

 we can ever hope to obtain of the direction taken by those 

 glacial currents must be derived from a study of the dis- 

 tribution of the materials they have transported. At present 

 this subject is not well understood and needs long and 

 careful study ; but a few facts, however, have been brought out 

 with considerable clearness during the last two or three years 

 of the Geological Survey. Among these are the known dis- 

 tribution of red quartzite, granite, and limestone boulders 

 before explained; but we have observed other facts that do 

 not now admit of so complete an explanation as these do, 

 and yet further investigation will doubtless cause them to be 

 as clearly understood as the others. This refers especially to 

 the discovery in the drift of substances that, however well 

 satisfied we may be as to the places of their origin, we do not 

 know it so positively as we do those of the boulders before 

 mentioned. 



JVative Copper has been found in the drift in various parts 

 of the State. It occurs in irregular lumps of a few ounces or 

 a lew pounds in weight. One lump found in Lucas county 

 by Col. W. S. Dungan, of Chariton, weighed upwards of 

 thirty pounds. These specimens are in all respects like the 

 native copper of the Lake Superior mines, and this region 

 is the only one known to us in which they could have 

 originated. If they did originate there the fact implies the 

 existence of a glacial current, during some part of the Glacial 

 epoch, having a southwesterly direction, and at right angles 

 with the one supposed to have coincided with the eastern 

 drainage of Iowa. 



Lead Ore. Fragments of the common sulphuret of lead 

 have also been found in the drift, but these are quite rare. 



