438 THE GEOLOGY OF MINNESOTA. 



[Cretacaout bed. 



limestone on the west side of the river at Ryan's quarry are deposits, one of white, and the other 

 of red clay, each two to three feet thick. The west end of White & Curtis' quarry is covered by 

 Cretaceous accumulations which are in turn overlain by drift. The section from top to base of 

 the bluff here is as follows: 



Section in ilix, bank of the Big Cobb river, N. E. J of sec. 19, Decoria. 



1. Yellowish sandy till 10-15 ft. 



2. Dark bluish till 30 ft. 



3. lied and yellow clay, seen at two places, each having an extent of only a 



few feet 2 ft. 



4. Ferruginous, sandy shale, with much interstratifled loose sand, some of 



these beds being mainly white, others dark, while the greater part have 

 an iron-rusted color, and are more or less cemented by limonite; visible 

 along a distance of 25 rods, from the extremity of the Shakopee lime- 

 stone southwesterly to the ford and foot-bridge; in thickness, about 10 ft. 



6. Incoherent, irregularly stratified sandstone, straw-colored or nearly white, 

 containing infrequent specks of a snowy white powder; exposed at 12 

 to 18 feet above the river, for a distance of only 25 feet, being ob- 

 scured below and elsewhere by the fallen talus 6 ft. 



6. Shakopee limestone, farther east rising 35 feet in a perpendicular cliff from 



the river, here 10-15 ft. 



Numbers 3, 4 and 5 are believed to be Cretaceous, but no fossils were seen in any of these 

 strata. 



Analyses of Cretaceous clays from the vicinity of Mankato. 



Five analyses, shown in the table below, have been made for this survey, of samples of the 

 very fine, more or less clayey silt which has been described in the foregoing pages in respect to 

 its manner of occurrence. 



The first of these analyses (No. 67, eighth annual report) was made by Prof. S. F. Peck- 

 ham, and is the clay or shale filling hollows of the Shakopee limestone in the west part of Man- 

 kato. Prof. Peckham remarks: "Its composition places it with orthoclase, although it has the 

 physical properties of kaolin. It is chemically a slightly decomposed feldspar, while it has the 

 appearance and some of the properties of clay. It, however, appears to contain too much iron to 

 admit of its being used for white ware, although a practical test is often required to definitely 

 settle the value of clays for such purposes." 



The second analysis (No. 75, tenth annual report) was by Prof. J. A. Dodge, and is from a 

 nearly white clayey bed of considerable extent, which has been tried unsuccessfully for brick- 

 making, near the quarry of J. E. Beatty & Co., in section 20, Lime. "This was pulverized, 

 without grinding up the particles of gritty matter that were to some extent intermixed with it; 

 the powder was then mixed with distilled water, the suspended portion poured off and allowed 

 to settle for a day or two; the settled portion was then collected, dried at 212, and submitted to 

 analysis by the common methods for silicates." 



The third (No. 138, twelfth annual report) was by Mr. C. F. Sidener, and is a nearly white, 

 very fine-grained, somewhat friable earth, in the lower part of the succession of Cretaceous strata 

 in section 35, Mankato (from the east bluff of the Le Sueur river close above the railroad bridge, 

 in No. 4 of page 435). 



The fourth (No. 139, twelfth annual report), by Mr. Sidener, is from the same locality with 

 the last, and is the red ochery clay which was mentioned on page 436. 



The fifth analysis (No. 146, twelfth annual report), also by Mr. Sidener, is the clay or shale 

 observed between the Shakopee limestone and the Jordan sandstone in the L'lluillier mound 

 (No. 4, page 430). Like No. 1 of this table, but in less degree, "it is rather remarkable for con- 

 taining so much potash, which probably exists in it in the form of finely divided potash feldspar." 



1. 2. 3. 4. 5. 



Silica, Si O 2 70.10 8770 93.65 73.34 68.70 



Alumina, Al, O 3 16.99 7.24 2.15 14.75 18.04 



Lime, CaO 0.67 0.20 0.28 1.24 



Magnesia, MgO 0.07 0.12 0.05 0.56 



Potassa, K a 10.69 0.49 traces traces 5.28 



