STRUCTURE AND COMPOSITION OF THE TRENTON. 357 



section by reason of the generally uniform composition and structure 



throughout and the absence of well defined and constant stratigraphic 

 elements. 



The lower contact zone just mentioned gives in part areas of calcium 

 and magnesium carbonates and in part clusters of kaolinic material and 

 grains of quart/,. The general aspect is that of a rock whose original 

 characters have become in great part obscured by infiltration of new 

 material. 



The next layer above this contains hands of quite pure calcium car- 

 bonate often several inches thick. Scattered through these bands are 

 occasional clusters of pyrite and granules of carbonaceous matter. The 

 argillaceous bands which alternate with these show a finely crystalline 

 granular matrix, in which lie rhombohedrons of calcite (see figure 5, 

 plate 12). This layer readily crumbles on exposure to the air, causing 

 the compacter limestone bands to separate. The chemical composition 

 of this layer and that of other portions associated with it will soon be 

 given, when conditions will be seen which explain the crumbling and 

 great lack of cohesion which the rock presents. The layer above this 

 fails to show the banded character just mentioned, but its proportion of 

 alumina, silica and ferric oxide is also very marked. Within it pyrite 

 often becomes clustered in quite large nodules, and the cavities from 

 which the fossils have been absorbed contain on their walls incrustations 

 of beautiful calcite and pyrite crystals, both single and clustered. Well 

 developed rhombohedrons also characterize many portions of this layer* 

 Nowhere have the writers observed the presence of twinning in these 

 rhombohedrons, although cleavage is usually distinct. 



Chemical Composition. — This has always been a matter of great interest 

 to those who have examined the formation Owen, who called the 

 Trenton the ''Saint Peter's shell limestone" from its richness in organic 

 remains, stated that the lowest bed contained 65 per cent CaC0 3 and 13 

 per cent MgCO, and pronounced it a poor hydraulic limestone.t Many 

 analyses have been made in the chemical laboratory of the university of 

 .Minnesota. These analyses represent particularly the layers which have 

 some economic value, especially for building stone, for which the rocks 

 of the lower calcareous division of the formation are largely used. The 

 following table contains those of present interest : 



*C. W. Hall, Lithological characters of tli- Trenton lim istone, etc: Hull. Minn. Acad. Nal Sci 

 vol. iii, ii" 1. 1889, |>. 1 Is. 

 •{•Geology of Wisconsin, [owa and Minnesota, 18-12, \>\>. 71, 72. 



