114 Trenton Limestone of Minneapolis and St, Paul — Hall. 



siderable extent, an explanation of its origin could more easily be 

 made. The most probable supposition is that at the close of the 

 St. Peter epoch a deposition of shallow sea — argillaceous — material 

 took place as the sea bottom subsided and the change from a shore 

 line of sands to a deep sea area, with its varied molluscan and crus- 

 tacean fauna was brought about. After this formation of clay 

 had occurred, and in the deep sea, cavities and canals seem to have 

 been formed in the clay and calcium carbonate, with some 

 impurities, was infiltrated and packed into the pockets formed, thus 

 becoming the replacing material . So the line of contact is a very 

 uneven one, and fairly represented by figure 2, Plate I. which 

 shows a hand specimen 3x4 inches in size. 



The limestone spoken of as lying directly upon this argilla- 

 ceous and corroded layer is quite crystalline. Yet its texture is 

 uneven. For a thickness of about 15 feet, there is a constantly 

 recurring alternation of layers of tolerably pure calcium carbonate 

 with other thin and interrupted layers of a shaly and argilla- 

 eeous material. The last named series of layers possesses a darker 

 and bluer color than the former; it lacks as well the crystalline 

 habit, the firm compact appearance, and the real hardness and 

 toughness under the hammer. These thin layers weather and de- 

 compose more rapidly than the others when the rock is exposed in 

 the walls of buildings, the piers of bridges and other places where 

 the air and rain and frost Imve free access to it. Nearly all -the 

 older buildings of the two cities, built of this stone, which have 

 stood for some years, show a very marked corrosion of this portion: 

 indeed to such an extent has the corrosion gone in the oldest of 

 them, that blocks standing on edge have not infrequently split 

 apart along these lines of easy cleavage and large pieces have fallen 

 from their place, a suggestion to builders always to place blocks 

 of this stone in walls in the same horizontal position they occupy in 

 the quarry. This layer commonly called the "building stone 

 layer" carries but few fossils, although it must have been largely 

 formed from them. The molecular alteration the limestone has 

 undergone, has served almost entirely to obliterate the fossils at 

 this quarry. Occasionally an Endoceras magniventrum,Hallor E. 

 rapax. Hall is exposed, as the quarrymen separate horizontally the 

 blocks of stone along the blue argillaceous bands. 



The chemical and lithological characters of this stone will be 

 considered on a subsequent page. 



