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



up into a dissolved portion and a collection of clear, smooth and 

 well rounded quartz grains, etc. A thin section was made and it 

 showed the rock to be an arenaceous limestone. Doubtless it i.s 

 magnesian, as all our Minnesota limestones of Cambrian age are 

 strongly so; yet it is assumed that calcium carbonate predominates 

 since effervescence begins and proceeds rapidly in cold acid. Nu- 

 merotiS grains of quartz are embedded in this carbonate matrix; 

 they are all well-rounded and filled with liquid inclusions like 

 those of any ordinary sandstone. Figure 5, Plate I, is a fair rep- 

 resentation of the thin section magnified about 35 times. 



The conclusion from the borings of the two wells named is 

 that the upper portion of the uppermost layer of the Cambrian 

 dolomitic limestone, or dolomites of Minnesota contains a pro- 

 portion of silica in the form of rounded grains introduced in a 

 very natural way at the time of transition from a period of lime- 

 stone or dolomite formation to one of sandstone building. 



As a summary of these descriptions we can note: The general 

 stratigraphic characters of the Lower Silurian in the neighborhood of 

 these two cities are very persistent. The St. Peter sandstone every- 

 where underlies the Trenton; a band of arenaceous shale lies along 

 the contact of the two formations; the Trenton is an impure lime- 

 stone constantly interrupted in its lower portion by argillaceous 

 bands and giving place in its upper portion to a rock less distinct- 

 ly stratified and more f ossiliferous . 



In chemical composition it is seen that the purest bands of 

 the building stone layer contain only about 83 per cent . of calci- 

 um carbonate and nearly 6 per cent, of silica. The upper portions 

 of the formation carry as low as 40 per cent, of calcium carbonate 

 and magnesium carbonate combined. This diversity of contents 

 produces a rock which poorly resists weathering and rapidly disin- 

 tegrates when exposed to the air. 



Consequently the formation gives a poor quality of building 



material for general use. Its comparatively rapid distruction in 



the air and its low resistance to crushing force are noticeable; 



these qualities, taken together with the dull, bluish gray color, 



yield a building material which h as in Minnesota many superiors. 



remarkable rock description. The material iu the University museum 

 shows a rock too fine for a gritstone, (Dana's Manual of Geology, p. 65) and 

 it is not a felsite. (ibid, p. 71). The cement is not amorphous nor is it 

 feldspathic. The "red quartzite" of Baraboo is pronounced by Professor 

 Irving to be a non-granular rock, Geol. Wis. vol. 2. p. 505: the material un- 

 der discussion is decidedlv ijranular. 



