140 THE PENOKEE lEOX-BEAKING SERIES. 



is due to the presence of these layers of chert. The easternmost point at 

 which any rock has been seen that can be with certainty referred to the 

 limestone meml)er is near the northeast corner of Sec. 20, T. 47 N., R. 44 

 W., Michigan, where chert and chert Ijreccia are struck in a test pit a few 

 feet south of exposures of the siliceous slate. 



Origin of the limestone and chert. — From the statement already made, it 

 is apparent that no facts have been obtained going to show that the lime- 

 stone and the major portion of the chert of this belt are other than original 

 water deposited sediments. Whether the carbonates are of chemical or 

 organic origin we have no definite ])roof. Early in the study of these rocks 

 it was thought that they were chemical sediments, as we had then little or 

 no evidence of life independent of the nature of the rocks themselves and 

 the iron carbonates at higher horizons. Later, as other proofs of life accu- 

 mulated, the assumed chemical deposition of these carbonates became more 

 and more doubtful. Many geologists hold that such carbonates are evidence 

 of and could have been produced only by life agencies. Where there are 

 no fossils, as in these cai'bonates, it is at best a matter of opinion as to their 

 origin; but in making up a judgment upon this point, the organic beds of 

 chert of later times associated with limestones, and the beds of iron carbonate 

 and carbonaceous material in higher horizons of the series, are facts to be 

 taken into accolint. 



The vein-like character of a part of the chert implies that the silica has 

 been rearranged to some extent, or partially introduced subsequently t(? the 

 deposition of the main body of the belt. The deposition of similar cherty 

 carbonates of great thickness is definitely known to occur in the Carbon- 

 iferous and Permian periods.^ The chert is here probably all of organic 

 origin. Whether the chert in the limestones under discussion is an organic 

 or a chemical substance it is impossible to say ; but it is certain that in later 

 time we have the exact analogue of the deposits described which are def- 



' Ou the Organic Origin of the Cliert in the Carboniferous Limestone Series of Ireland, and its 

 Similarity to that in the Corresponding Strata in North Wales and Yorlcshire. George .Jenning.s 

 Hinde. Geid. Mag., London, New Series., decade III., vol. iv, )))>. 135-446. On the Chert and 

 Siliceous Scliists id' the I'erino-Cavlioniferous Strata of Spitzbergen, and on the Characters of the 

 Sponges therefrom, which liave been described by Dr. K. von Dunikowski. Dr. Hinde, ibid, vol. v, 

 pp. 241-251. 



