AND COAL-MEASURES U F 10 W A. 



105 



to return after making the astronomical observations necessary to determine our 

 position, which was found to be in latitude 42° 39' 06". 



The preceding sketch conveys a very correct idea of the appearance of this desolate 

 region of drift knolls, as seen from one of their summits, with the wilderness of 

 prairie stretching away to the northwest. 



There remains to be said, in this place, only a few words with regard to the 

 extreme western branches of Red Cedar, viz., Beaver and Otter Creeks, which were 

 explored by Mr. Macy, with a view to ascertain the precise eastern boundary of the 

 carboniferous rocks on the Iowa River. 



On account of the low, wet country bordering these streams, it was only at a few 

 points that the rocks were visible, and these only in low ledges, near the water-level, 

 or in partial protrusions from under the mass of drift in the higher ground. 



Mr. Macy succeeded, however, in detecting, in a ridge about five miles north of 

 Otter Creek, a flesh-coloured limestone, containing that variety of Leptcena depressa 

 of Dalman, which occurs in the Corniferous Limestone of New York, and is known 

 there under the name of Strophornena uudulatus. At a higher level in the same 

 ridge a buff-coloured magnesian limestone was observed, in which no fossils were 

 discovered, but which possesses the lithological character of the dolomitic beds of 

 the carboniferous limestones found near the Iowa, a few miles below the Rapids, 

 near latitude 42° 30'. Thus, the dividing ridge between the head waters of the 

 extreme western branches of the Red Cedar River (Otter Creek and Elk Fork), is 

 probably the boundary between the limestones of the Devonian and Carboniferous 

 Eras. 



In consequence of a severe intermittent which Mr. Macy contracted while travers- 

 ing the low fens of this section of Iowa, exposed to the intense heat of a July sun, 

 he was prevented from prosecuting his investigations further north. 



Carboniferous Bocks of the Des Moines, below the Raccoon Fork. — The first good 

 section exposed on the Des Moines is at St. Francisville. The Archimedes lime- 

 stone occurs here, a few feet above low-water mark; and upon it rest several 

 members of the Carboniferous Limestone, in the order represented on Section 

 No. 4, D * 



The geodiferous beds which immediately overlie the Archimedes limestone, are 

 here mostly concealed by the alluvion of the Des Moines, and the talus from the 

 superior strata. The St. Francisville quarry affords, however, a good opportunity 

 of examining the buff-coloured rock, which lies about fifty feet above the bed of the 

 Des Moines, with a face of about ten feet exposed. It is at present obtained here, 

 for the construction of the public works in the vicinity, in large blocks, from two to 

 three feet thick. This rock is of a decided dolomitic character, yielding by analysis 

 thirty-seven per cent, of carbonate of magnesia, and fifty-six per cent, of carbonate 

 of lime. Its specific gravity is considerably below the average of common lime- 

 stones, being but 2 "589 ; a property which must be considered an advantage, 

 regarding it as a building material. Its durability has not been fully tested; but 



* The Sections in this chapter designated as No. 1, D, No. 2, D, &c, are the Sections on the Des Moines 

 River. 



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