CAKBONIFEROUS LIMESTONES OF IOWA. 



91 



2. The encrinital group of Burlington, with its partings of red, ferruginous clays 

 and marly earth, together with its substrata of oolitic and hydraulic limestones. 

 This group terminates upwards in nearly white, semicrystalline, and semi-oolitic 

 limestones. Several of the beds of the division afford materials for building. The 

 oolitic bed has been much used for sills. 



3. Next comes a great mass of reddish-brown limestones, alternating with 

 numerous thin beds of chert, and many ramifications and reticulations of the same 

 mineral. The mural escarpment below the town of Hannibal, is composed of the 

 calcareous series comprised in this subdivision. Dimension-stones of considerable 

 size can be obtained in these beds when sufficiently free from chert. 



4. The latter division passes upwards into the gray, cherty limestones, which 

 form the wall of rock washed by the Mississippi, below the Keokuck Landing. 



5. Next come the so-called shell-beds of the Rapids of the Mississippi. They claim 

 this appellation in a double sense, — both because they are densely crowded by fossil 

 shells, and because many layers are schistose and " shelly" in their structure. 



6. The uppermost member is appropriately termed Archimedes Limestone, in- 

 asmuch as this is the most remarkable and characteristic of its organic contents, 

 though by no means the only fossil which it contains. 



Both of these divisions afford some building material, but most of the beds are 

 too schistose. 



In the upper series, six distinct members may also be recognised : 



1. At its base we have beds somewhat schistose and argillaceous, especially cha- 

 racterized by enclosed Geodes, lined with crystals of quartz and calcareous spar. 

 These beds disintegrate by exposure, and therefore afford but indifferent building 

 material. 



2. Over these usually lie thick beds of buff magnesian limestone, either of cel- 

 lular structure, or with minute tubular ramifications running through it ; often with 

 green particles disseminated. These limestones, when thick-bedded, as they often 

 are, supply good building material. 



3. This member is usually composed of sandstones, separated from the former 

 either by thin bands of chert or by argillaceous partings, and with fine gravel dis- 

 seminated. It yields tolerable building-stone. 



4. The lower concretionary limestone, close-grained, light-coloured ; its concre- 

 tions often surrounded with earthy matter and marly clay; sometimes evenly 

 bedded, and then affording building-stones, which are seldom, however, of large 

 dimensions. 



5 and 6. Above the foregoing are again repeated gritstones and concretionary 

 limestones. No. 6 is almost invariably concretionary in its character, with argilla- 

 ceous, shaly beds of the coal-measures resting immediately upon it. 



The better to enable the student of American geology to draw comparisons for the 

 identification and parallelism of strata, I have embodied the principal facts and 

 details in the following combined table and section, on the next page. 



The approximate average thickness is given in this table, rather than the 

 absolute thickness at any particular locality-. 



