HISTORICAL SKETCH. 



The Galena, with a thickness of about 160 feet, reposes on the Upper Blue beds, in 

 southeastern Wisconsin. This is another dolomyte, but sometimes has siliceous and 

 aluminous matter in considerable amount, some of its thin layers, or partings consisting of 

 shale. It is in general heavy-bedded, irregular, coarse-textured, gray or burt, with fre- 

 quent crystal-lined cavities, but toward the north becomes more shaly, and has a greenish 

 or bluish color, with more fossils and sometimes a strongly graptolitic fauna. 



The Cincinnati shales and limestones, next in ascending order, have an approximate 

 average thickness of 200 feet. The clay shales and limestones prevail in the upper por- 

 tion, and slaty and arenaceous shales in the lower. Yet in the northern part of the area 

 limestone prevails in the upper part over the shales. The characteristic faunal feature is 

 the prevalence of corals and bryozoans. Upwards of 30 species were collected from the 

 shale thrown out of two shafts of no considerable depth. Brachiopods are next in 

 abundance, Orthis and Strophomena predominating. 



In recapitulation of the facts of tha Trenton period Prof. Chamberlain draws three 

 conclusions respecting its paleontology, viz: (1) There are a considerable number of 

 species that range throughout the whole Trenton period, including the Cincinnati epoch, 

 and are therefore of no service in discriminating between its subdivisions; (2) There is 

 another portion whose occurrence is chiefly confined to the strata of the Trenton epoch; 

 and, (3) There are a few that are not authentically known to occur either above, or below 

 the Galena, and may be regarded as characteristic of it. Of this number Receptaculites 

 oweni and Murchisonia bellicincta or major, are the most constant and reliable. Lingula 

 quadrata, although rare in other beds in eastern Wisconsin, does not appear to be 

 strictly confined to the Galena. Pusispira ventricosa and F. elongata are perhaps to be 

 added to this list. 



The tables given by Prof. Chamberlin showing the stratigraphic distribution of 

 species of the Trenton period distribute the species that had been identified in the 

 following manner: In the Lower Buff, 53; Lower Blue, 57; Upper Buff, 69; Upper Blue. 

 25; the Trenton epoch, 195; Galena proper, 62; Galena modified, 88, and the Cincinnati, 66. 

 Total in the Trenton period, 295. 



R. D. Irving. 



1878. In the same volume as the last Prof. R. D. Irving described the "Geology of 

 Central Wisconsin." A portion of his chapter is devoted to the Trenton and Galena 

 limestones, the latter occuring, however, in unimportant, isolated cappings. The 

 Trenton, as described, embraces two parts, the Lower Buff (or "the Buff"), and the Lower 

 Blue (or "Blue"). The former is generally a dolomitic limestone about 25 feet thick, the 

 latter an argillaceous limestone with but little carbonate of magnesia, the interleaved 

 calcareous dark shale sometimes containing black graptolite-like markings near the base. 

 The thickness of the Blue is not given. 



