IOWA ACADEMY OF SCIENCES. 55 



The degree of induration, the pronounced jointing, and the 

 general ancient aspect of this conglomerate render it rea- 

 sonably certain that it is not a part of the drift which overl es 

 it. But it has pebbles of Archsean rock, and one of these was 

 nearly six inches in diameter. None of them were observed to 

 be scored, though quite a number were examined. The aver- 

 age size of the pebbles is from one-fourth to two inches in 

 diameter. On the other hand, it is not believed that it can 

 belong to the coal measures. Some of the pebbles appear to 

 be pieces of coal measure concretions and lumps of coal meas- 

 ure clay, and the aggregation of rocks represented in the 

 pebbles is unlike anything observed in the coal measure con- 

 glomerate. For comparison, a collection of fifty pebbles was 

 made, representing the average sizes. The proportion of 

 specimens of dilferent rocks in this lot was as follows: 



Yellow chert 32 per cent 



Greenstone 26 " 



Granite {mostly red) 10 " 



White quartz (some of a faint, pink color) 8 " 



Fragments of coal measure rock 4 " 



Light red orthoclase 2 " 



Black felsite 2 " 



Porous Niagara chert 2 " 



Chalcedony 2 " 



Orthoclase-biotite rock 2 " 



The only conclusion which can at present be drawn, as to 

 the age of this conglomerate, is that it is post- Carboniferous 

 and preglacial. Dr. Calvin, who has seen it recently, pro- 

 nounces it identical in nature to the Rockville conglomerate 

 described by McGee. It also somewhat resembles the Cretaceous 

 conglomerate found in Guthrie county by Mr. Bain. Possibly 

 it may be an outlier of the Lafayette formation, observed far- 

 ther, south by McGee and by Salisbury. 



In the south bluff of West Hill in Muscatine, just east of 

 Broadway street, there lies on top of the coal measures and 

 under the drift a small remnant of a conglomerate somewhat 

 resembling that above described. It is seen for a distance of 

 only three or four rods and its greatest thickness is three feet. 

 It is plainly unconformable with the beds below. The base is 

 a very pebbly sand, held in a dark ferruginous matrix, which, 

 in some places, does not wholly fill the interstices between the 

 pebbles. The upper surface is a brown ferruginous, moder- 

 ately fine sandstone of about the same hardness and aspect as 



