GALENA LIMESTONE. 307 



ified by the change of conditions which this depression caused, aid- 

 ed by the protection which the projecting axis above mentioned and 

 its associated reefs aiforded. The change in the Galena limestone is 

 gradual and progressive for 40 or 50 miles, beyond which its nature 

 as modified becomes constant for nearly a hundred miles to the lim- 

 its of the state. 



Organic Contents. For a complete list of the fossils found in the 

 progress of the survey in this formation, the reader is referred to the 

 general table of fossils of the Trenton, Galena and Cincinnati forma- 

 tions, where may also be obtained a convenient view of their distribu- 

 tion, and for a knowledge of their special distribution and associa- 

 tions, reference may be had to the lists given in the local descriptions 

 that follow. In addition to these, only a few general remarks need 

 here be made. The " Lead Coral," Receptaculites Oweni, is the most 

 characteristic fossil, and, from the ease with which it can be distin- 

 guished, furnishes a most convenient and reliable guide. It is not, 

 however, always to be found in limited exposures of the formation, 

 though it is very widely distributed. A large coiled shell, having a 

 high spire, known as Murchisonia bellicincta is almost equally 

 characteristic in this region and is more abundant. This species is 

 regarded as identical with Jbfurchisonid major, whose typical locality 

 lies in this horizon in the Green Bay region. Two somewhat similar 

 fossils, Fusispira ventricosa and F. elongata are, so far as yet deter- 

 mined in this region, confined to this formation. Lingula quadrata, 

 a phosphatic shell, is also regarded as characteristic, though it 

 is not strictly confined to this horizon. Several other species, so far 

 as present knowledge goes, are found only in these beds, but their 

 distribution is not sufficiently general and well ascertained to justify 

 regarding them as distinguishing species. The majority of the spe- 

 cies, as we should expect, are also -found in the adjacent formations. 

 Of the 75 species collected from this formation, setting aside doubt- 

 ful forms, 19 are confined to it, 42 are also found in lower strata, but 

 not above, and 14 are found both below and above. These state- 

 ments relate only to the collections made, and it is to be considered 

 that owing to practical difficulties, the collections from the adjacent 

 strata above are less full than from those below, and that from the 

 immediately superjacent beds there are no collections, because no ex- 

 posures in this region. 



Thickness. The average of several estimates gives this formation 

 a thickness of about 160 feet, with a variation from about 125 feet to 

 200 feet. 



Industrial value. While this is the formation which is so pro- 



