Trenton Limestone of Minneapolis and St. Paul — Hall. 119 



cal analyses, any entire area might be taken for dolomite rather than 

 calcite, were this character of the mineral constituents to be relied 

 on as a distinguishing one, as Zirkel* suggests may be done. 

 Figure 3, Plate I is an attempt to represent the texture of this 

 rock. The rhombohedral cleavage is nearly universal. 



As the bands of shaly and argillaceous material are reached 

 argil laceous and silicious substances enter into the composition of 

 the rock as quite constant constituents. The former appears as 

 an earthy matrix holding rhombohedrons of calcite. The sili- 

 cious constituent lies partly in the argillaceous mineral and partly 

 in the small grains of quartz which polarize brightly and are well 

 filled with fluid cavities. These grains are all well rounded. 

 Pyrite is seen in every part of the formation. It occurs frequent- 

 ly in segregations as shown in Fig. 4 c, Plate I, and almost every- 

 where in minute crystals, sometimes cubes and sometimes pyrit- 

 ohedrons. They are most plentifully grouped around the absorbed 

 fossils, and occasionally with the transparent, coarsely crystalized 

 calcite, they fill the cavities thus formed. 



In certain minor respects the layer immediately above the 

 building stone layer differs from the description just rendered. A 

 large proportion of argillaceous material is present. Where fos- 

 sils once were, absorption has taken place, and beautiful but small 

 (•rystals of calcite fill the cavities left. Sometimes the calcite is 

 accompanied by segregations of pyrite crystals, also small but well 

 formed. Both calcite and dolomite are held to appear in the more or 

 less transparent grains, although no traces of twinning are to be seen. 

 All the individuals of both minerals are sharply and uniformly 

 rhombohedral in habit. Pyrite appears in segregations too fine 

 to be detected with the unaided eye. See Fig. 4, Plate 1. 



The upper and weathered layer at this quarry presents but 

 little of lithologic interest. The microscopic structure of the 

 compacter bands is very like that of the layer below. At other 

 points, however, as at the Maloney quarry, there is a band in this 

 layer which has a thickness of only a few inches, but of a hard- 

 ness far in excess o£ any other band in the whole quarry. Since 

 there is but little save the carbonates of calcium and magnesium 

 in this band, the hardness must lie chiefly in the compact condi- 

 tion of the stone. There is certainly more cohesion in this than 

 in the more heterogeneous portions of the rock. 



*Mikr()sk()pisclio IJeschafFfMilH-it dor Min<'rali(Mi iind Gosteine, 187,% p. I»95. 



