324 Loiver Silurian of Wisconsin and Minnesota. 



not determined. I saw the lowest strata of the Galena at Evans- 

 ville, Dodgeville, etc., in Wisconsin, and formed the opinion that 

 the corresponding line in Minnesota, should be drawn at the top 

 of the Orthisina bed : but the fauna here seems to demand a divis- 

 ion at the top of the Zygospira bed or below it. 



From the top of the Orthisina bed to the upper limit of the 

 Trenton group is fully 100 ft. in Minnesota, and in many places 

 the rock is exposed in vertical walls, for part or all its thickness. 



The fauna in the Galena formation is much more meager 

 than in Trenton, and for that reason close division into beds is less 

 easily made and with less certainty. Three subdivisions however 

 have been noted in Minnesota. 



1. The first of these is a somewhat carbonaceous limestone, 

 about thirty feet thick, that crumbles more or less in weathering. 

 The name Camarella bed seems appropriate on account of several 

 species of that genus, which occur here and have not been found 

 in other beds. 



2. The next twenty feet is of a firm, very durable limestone 

 with few fossils of several species. Inarticulate brachiopods are 

 well preserved. From the occurrence here of Lingulasma schu- 

 cherti(f) Ulr., the name Lingulasma bed has suggested itself. 



3. The last fifty feet of the Galena formation, which I shall 

 call the Maclurea bed, is characterized by large gastropoda — 

 Murchisonia major Hall, Fusispira elongata Hall, Maclurea 

 cuneata Whitf., Rapliistoma lenticularis (large variety). 



This Maclurea bed I feel safe in correlating with the last de- 

 posited strata of the Galena formation in Wisconsin, both on lith- 

 ologic and palseontologic grounds. And indeed no reason is 

 known to me for supposing that the strata composing the Trenton 

 group in Minnesota and Wisconsin were not in every case con- 

 tinuously and contemporaneously deposited. As to the first and 

 the last bed, identical fossil forms and similarity in structure, to- 

 gether with nearly continuous exposures, furnish sufficient evi- 

 dence of their relation on both sides of the Mississippi valley. 



The succession of strata through the Trenton group in Min- 

 nesota forms a gradual transition from limestone to typical shale 

 and again back to limestone. In Wisconsin this transition is less 

 and less marked in proportion to the distance from the northwest- 

 ern extension of the strata in Minnesota, i. e. from the line of the 

 advancing and retreating shore of the sea in which the Trenton 

 sediments were deposited. 



