1 l*i Trenton Lhnesfone of .Af'ninenpoli^ and St. lAntl Jfalf. 



hoisted for some yejirs, but recently the Saint Paul, Minneapolis 

 4ind Manitoba railway has worked it extensively, in securing 

 material for the great viaduct across the river from the upper end 

 •of the quarry to the Union depot. The layers suitable for working 

 ?ftand up above the level of the river, so that the drainage of the 

 -tfuarry isall that could be desired, and the horizontal position of 

 the stone and the pronounced vertical joints conspire to imike 

 -the quarrying of the rock highly satisfactory. 



The description below and the figure following it will serve 

 to make clear the thickness and relative position of the different 

 layers constituting the so-called Trenton limestone at this quarry. 



Thr Undekt.ying St. Peter Sandstonk. — Beginning at the 

 3>ottom we note that a bed of St. Peter sandstone underlies the 

 limestone beds of the quarry. This sandstone has a thickness at 

 this city of 1(U feet, according to the record furnished by Col. J 

 B. Clough, former city engineer.* Yet the river at the quarry is 

 not more than 30 feet below the bottom limestone layer. This 

 sandstone shows a medium texture throughout its whole extent, 

 the grains are thol'oughly rounded and of very uniform clearness 

 and purity.** The rock is extremely friable: rarely does one find 

 ;a block sufficiently cemented to hold its own weight when over- 

 turned, save where at the edge of the gorge, a cement of calcium 

 <iJirbonate has become infiltrated. A spoonful of the sand plunged 

 in a glass of water produces a cloudiness caused by a fine and 

 Avhite kaolin; it is doubtless this kaolinic constituent occcur- 

 ring all through the stone which causes the marked friability 

 already mentioned, by preventing the cementing of the quartz 

 grains with the silica cement carried by the water percolating 

 through the formation. 



The color of the sandstone varies with the local conditions, 

 ill though as a rule in this vicinity the prevailing color is white. 

 Where any other color is taken on, it is due to the infiltration of 

 coloring matter from the overlying rocks, and is not seen to pene- 

 trate the quartz grains themselves . The percentage of impurity 

 in this sandstone is so small that it has been thought that a good 



*These Bulletins, vol. I, p. 187. 



**In this formation Chamberlin notes the medium coarseness of the quart- 

 zose grains and its comparatively freedom from that adtuixture of coarser and 

 fiuer material which is common in sandstone. Geol. Wis. vol. i, p. 145. 



