260 APPENDIX. 



intermixed with the styhna. Its value as a marble may be 

 considerable, should it be obtained in blocks of sufficient size. 

 In polishing, however, the organic structure of the coralline 

 causes cellular imperfection on its surface. 



" MILLSTONES. 



" In section twenty-two, township eighty-nine, range three 

 west of the fifth principal meridian, the United States sur- 

 veyors had reported a ' millstone quarry.' There seems, 

 however, to have been no better foundation for this report 

 than the presence of some granite boulders, very numerous 

 on the northern portion of the eastern boundary of the dis- 

 trict, and also throughout the western ranges of Iowa. 

 These erratic boulders constitute a peculiar feature in the 

 prairie scenery, and are often of great size. One was 

 reported to me by a sub-agent, somewhat indefinitely, as 

 being ' as large as a steamboat.' A smaller one, afterwards 

 measured, was eight feet high, and ninety feet in circumfer- 

 ence. They are composed of granite, green stone, porphyry, 

 and other primitive rocks. 



" Similar boulders, in the State of Illinois, are, in default 

 of more suitable materials, sometimes employed to make 

 millstones ; but the labor of the manufacture from these 

 primitive rocks is very great, and a ' millstone quarry' of 

 such a character cannot be considered of value. 



" In the course of a geological reconnoissance of the State 

 of Indiana (which, as geologist of that State, I had, two 

 years since, occasion to make), I found good millstone quar- 

 ries in a rock formation which is the equivalent of that of 

 Wisconsin, and I hoped to make similar discoveries in the 

 course of this survey ; but I have seen no rock, either in 

 Iowa or Wisconsin, which combines hardness and porosity 

 enough to render it suitable for this useful purpose. 



