l893-] FAIRCHILD GEOLOGICAL HISTORY OF ROCHESTER, N. Y. 219 



shallow waters, a slowly subsiding sea bottom, and the degradation of 

 some land area of iron-bearing rocks, which should supply the 

 peroxide of iron that gives the red color. This is a remarkable 

 formation in composition, thickness and extent. The conditions 

 seem to have been such as are usually transitory, but which in this 

 case persisted for a vast time over considerable area. The shallow 

 depth was here preserved by slow subsidence, although slight changes 

 of conditions produced variations in the texture of the rock. 



The deposition of the red Medina sands ended with remarkable 

 abruptness, and subsequently, whether immediately or not is 

 uncertain, a fine greenish silt was filtered over the bottom, and this 

 continued until in this locality 24 feet were accumulated. Probably 

 this indicates a sudden increase of depth. By some physical change 

 the velocity of the currents over this locality was certainly checked, 

 either by increase of depth or by the raising of barriers. This shale, 

 called the Lower Green Shale of the Clinton, terminated as abruptly 

 as it began. The argillaceous was replaced by calcareous sediment, 

 and 14 feet of limestone accumulated, the Lower Clinton (or Penta- 

 merus) limestone. These sudden and frequent changes of sedimenta- 

 tion, from siliceous to argillaceous, from argillaceous to calcareous, 

 or the reverse, are difficult to explain for want of knowledge as to the 

 physical conditions of the surrounding regions. 



Near the base of the Lower Clinton limestone occurs a bed of 

 hematite iron ore of singular character. It is about a foot in thickness, 

 chiefly pure peroxide of iron, more or less minutely concretionary, 

 fossiliferous, and probably accumulated upon the sea-bottom as a 

 sediment. (*) In other areas of the Clinton the ore may have a 

 different horizon or position in the succession of beds, and in some 

 localities it is of considerable thickness. The source of this iron is 

 unknown. [^) 



When the lime deposition gave place to silt another 24 feet of 

 greenish clay were formed. In this Upper Green Shale of the Clinton 

 there are found at one horizon lenticular masses of pure limestone 

 wholly composed of the consolidated shells of a tiny Brachiopod 

 mollusk {LeptocKlid) which formed colonies at certain spots upon the 

 muddy sea bottom. These masses of molluscan limestone are 

 exceedingly interesting, and clearly prove and illustrate the organic 



(4) "On the Clinton Iron Ore," by C. H. Smyth, Jr., Amer. Jour. Sci., \'ol. XLIII, pp. 487-496. 



(5) All these strata, from the Medina upward, are clearly shown and are accessible for exam- 

 ination along the old road known as Buell Avenue, nortli of Seneca Park bridge, below the 

 lower falls, west side of the ravine. 



