SAG HALL AND SARDESON — PALEOZOIC FORMATIONS OF MINNESOTA. 



The shales of this scries are hut little known. Well borings at Man- 

 kato, Blue Earth city and elsewhere show shales with but little crystal- 

 Unity or coherence. They have a green color usually, which is possibly 

 due to the presence of ferrous oxide. Everywhere they are partly made 

 up of carbonates, with a liberal supply of quartz grains. 



The dolomitic beds have certain characters of lithologic interest. Along 

 the Mississippi river at Nininger, Hastings, Redwing, Frontenac and 

 elsewhere a marked porous condition is frequent. It is more character- 

 istic of the heavier layers. It is associated with concretions, with com- 

 pact, finely granular streaks, and with changes in composition in such a 

 way as to show undoubtedly the secondary origin of the dolomitic feature. 

 Ordinarily the vesicular structure is not coarse, yet it is readily seen with 

 the unaided eye. Locally the cavities are larger until a honey-comb 

 structure appears, or even until the material is wholly removed and a 

 cavernous condition results, with its recesses beautifully lined with sta- 

 lactitic incrustations. These seem to be of pure cal cite and are white. 

 Streaks of a limonitic color occur in the rock. 80 far as they were 

 examined, they were produced by the infiltration of ferric oxide, which 

 stains the surfaces of the grains and rhombohedrons which build up the 

 mass. As a rule, the conipacter portions of the beds are of a much 

 lighter gray color than the vesicular. Locally a greenish color pervades. 



Microscopically there are two persistent characters visible throughout 

 the series of specimens examined. The first is the rhomhohedral form 

 of the grains, manifested either in the external form of the individuals 

 or in their internal cleavage, or in both respects. The external outline 

 is, indeed, modified by the contact of neighboring particles, yet the be- 

 ginnings of all the individuals are constantly under the laws of rhom- 

 hohedral growth (see plate 12, figure 1, compact dolomite from Hastings). 

 In the coarser phases of the rock this crystallized condition is even 

 more pronounced than in the finer. In the vesicular portions not only 

 is the rock itself in this condition, but the cavities are lined with the 

 projecting angles of rhomhohedra. Where the texture is coarse and the 

 vesicular structure nearly wanting, numerous spaces occur where clusters 

 of perfectly formed rhomhohedra are gathered, and each figure has a 

 border of transparent material whose condition strongly suggests calcite. 

 Such a phase of the lower Shakopee occurs at Mankato, in the quarries 

 of the northern portion of the city (see plate 12, figure 2). The sample 

 was taken 25 feet above the Jordan sandstone. Again, a section taken 

 from the old quarry at Frontenac, on the Mississippi river 10 miles 

 below Redwing, shows the vesicular structure very pronounced. The 

 rhombohedral outline of the individuals is clearly defined, and by a seg- 



