336 HALL AND SARDESON — PALEOZOIC FORMATIONS OF MINNESOTA. 



The basal Conglomerates of the Potsdam,. — Everywhere so far as the basal 

 beds of the lower sandstone, or Potsdam as it is generally called (follow- 

 ing the suggestion of Professor James Hall in 1843)* have been seen 

 they are strongly conglomeratic. The conditions prevailing in the for- 

 mation of such basal conglomerates have been pointed put by Irving,f 

 and indeed such beds are precisely what should be expected under the 

 conditions actually obtaining in the northwest at that time: material 

 broken from sea-cliffs and knocked about on the beach of a slowly sink- 

 ing land area. 



At Minneopa the boring of a deep well in 1888 showed a conglomerate 

 lying between 575 feet and 800 feet below the surface. It was made up 

 of well rounded pebbles of a vitreous quartzite, some of them very large. 

 and many from one to three inches in diameter were thrown out during 

 the boring. They were bound together by a fine red or reddish-yellow 

 cement, which comes out of the well as an exceedingly clayey mud. The 

 material of the pebbles is identical in every respect with the quartzites 

 occurring from Courtland through Watonwan and Cottonwood counties 

 into southwestern Minnesota and southeastern South Dakota. It is 

 vitreous, non-granular, of varying texture and red color. A thin section 

 shows the cementing material to be arranged crystallographically with 

 the cemented grains and as enlargements upon them precisely as Irving 

 and Van Hise % have pointed out for the quartzites of Redstone (Court- 

 land), the nearest area of these rocks to the Minneapolis well, and also 

 as shown in a slide from the quartzite of Cottonwood county, Minnesota. 

 The Minneopa well was sunk to the depth of 1,000 feet, but the record 

 below 800 feet was thoroughly unreliable. § 



On Snake river two miles above Mora there lies a bed of horizontal, 

 cross-bedded conglomeratic sandstone. This exposure is less than three 

 miles from the Ann river knobs of hornblende biotite-granite. The 

 conglomerate has a cheerful light-pink color and is uneven in texture, 

 the largest pebbles reaching a diameter of two or more inches. Many 

 rounded pieces of feldspar are to be seen. The whole aspect of the rock 

 is that of a clastic worn directly from the granites lying in the neigh- 

 borhood. The pebbles are somewhat kaolinized, more so than are the 

 granites of central Minnesota at the present time, a fact suggesting thai 

 the great bulk of the erosion which these rocks have undergone has been 

 suffered since the beginning of Pleistocene time. Both Shumard || and 



♦ Natural History of New York, pari iv. Geology, 1843, p. -jt. 



fOn the classification of the early Cambrian and Pre-* lambrian formations, R. I>. [rving, Seventh, 

 An. Rep. U. S. Geol. Survey, 1886, p. 397. 



JOn Secondary Enlargements of Mineral fragments in certain rocks: Bull. U. S. Geol. Survey 

 no. 8. 1884, p. 34. 



j)Cf. Hull. Minn. Acad. Nat. Sci., vol. iii. no. 2, 1891, p. 250. 

 Owen'.- i ion logical Sur\ <-y of Wi.-., la. ami Minn.. 1852, pp, 524, 525. 



