TRENTON LIMESTONE. 295 



The average thickness may be put down at from twenty to twenty- 

 five feet. The limestone is varying in texture. Usually it is a mix- 

 ture of earthy material and minute crystals, but sometimes has a 

 compact crystalline structure, and occasionally a coarse granular one. 

 Some layers are little else than a mass of fossils. The color is bluish, 

 or grayish, weathering to light gray or buff. 



The shale is a bluish green, and aside from mingling with the lime- 

 stone somewhat, it forms seams and partings between the layers. 

 These are seldom over two inches thick in the southern part of our 

 province, but attain more considerable dimensions at the north. A 

 notable amount of carbonaceous material is sometimes associated with 

 this shale. It is usually very highly fossiliferous. 



In chemical composition the limestone layers do not essentially 

 differ, so far as tested, from the Buff' already described, being an im- 

 pure magnesian limestone. The analysis previously referred to in- 

 cluded only the limestone layers, the shaly partings being excluded. 

 Including these, probably one-third of the whole mass would be found 

 to be silicious and aluminous material. 



Besides the bluish green cast that the shale gives to the mass, it has 

 served to protect the limestone from the leaching action of percolat- 

 ing water, so that it also oftener retains its original bluish hue than 

 the beds below and above, and renders the name applied to it not in- 

 appropriate. It is characterized by a much greater abundance of fos- 

 sils than the beds below. These differ from those below in the 

 much greater abundance of Corals, Bryozoans and small Brachio- 

 pods, especially the Orthidse. Murchisonia gracilis appears in 

 great abundance near the base of this division. It is not found in 

 my somewhat extensive collections from the Lower Buff, which indi- 

 cates its rarity, though perhaps not absence, from that horizon. Bel- 

 lerophon ~bilobatus is very abundant, though not confined to this 

 horizon. This division comprehends, in greater or less numbers, a 

 large proportion of all the species found in the Trenton limestone of 

 this region. 



j. The Upper Buff Beds. This is the thickest and most important 

 subdivision of the group in the Rock river valley, reaching a vertical 

 dimension of fifty-five feet. It is less uniform in its several parts 

 than the two preceding, and is less easily described in general terms, 

 and the reader will perhaps find the detailed description of the sec- 

 tion at Beloit, given subsequently, under the head of local descrip- 

 tions, more satisfactory than the general statements here made. The 

 most prevalent kind of rock is a rather heavy bedded limestone, ob- 

 scurely banded and mottled with light gray and buff, giving the 



