SURFACE DEPOSITS. 85 



proportion of the bulk of the drift, but it is always a charac- 

 teristic constituent of it. 



Boulders constitute a very conspicuous and characteristic 

 feature of the drift, although they form but a small pro- 

 portion of the bulk of that deposit in Iowa, where its fine 

 materials are everywhere greatly in excess of all others. 

 The boulders of Iowa drift are composed of granite, quartzite, 

 and limestone rocks; those of granite being much more 

 numerous than all others in eastern Iowa, and those of 

 granite and quartzite both largely prevailing in the western 

 part of the State. Limestone boulders are comparatively 

 rare anywhere, but they are found most frequently in the 

 middle portion of the State as one traverses it from east to 

 west. The largest as well as the most numerous of the 

 boulders are those of granitic composition. Occasionally 

 these are quite large, reaching fifty tons in weight or 

 upwards, but they are usually very much smaller than this. 

 Although generally they have at least a somewhat rounded 

 form, they seldom present any real appearance of having been 

 water-worn as the pebbles have. Their rounded forms seem 

 due rather to the concentric decomposition of the broken 

 fragments of rocks, thus rounding off their angles; or to the 

 somewhat concretionary character of the mass of which it 

 was originally a part. This, it is thought, is particularly 

 true of the granite, which is known sometimes to contain con- 

 cretionary centres in which the rock is harder and more 

 compact than the greater part of the mass is. 



Rare Substances and Fossils are sometimes found in the 

 drift of Iowa. The fossils are always those of the older 

 rocks, none of any kind having been discovered in it that are 

 properly referable to the age of the drift itself. Lumps of 

 copper, lead-ore, and even traces of gold, have been found in 

 it. Also lumps of impure coal, pieces of wood, and other 

 traces of vegetation. All these have been transported and 

 are as much strangers in Iowa as the granite boulders are. 

 Their origin will be referred to on a subsequent page. 



Distribution of the Drift. No evidence of anything like 



