SECONDARY CHANGES IN THE ROCK.. 345 



that the dolomitization of* vast beds of Cambrian limestones and the eon- 

 sequent shrinkage in bulk is alone sufficient to account for the displace- 

 ments. 



The sandstones in all their phases have the usual characters of this 

 rock. They may briefly be summed up as follows : Structure, massive 

 and firmly bedded, with occasional shaly layers in local development ; 

 cross-bedding not infrequent ; in places, indeed, very strongly marked ; 

 texture, varying through every condition from the conglomeratic to the 

 finely comminuted ; composition, varying somewhat from clear quartz by 

 the occurrence of felclspathic and calcareous grains. A cemented condi- 

 tion of the grains in several counties furnishes a stone which is used for 

 building purposes,* although such cementing is nowhere found to be at 

 all extensive, and is no doubt due to an infiltration of carbonates from 

 an overlying layer of dolomitic rock. The ferruginous appearance seen 

 in places is due to infiltrated hydrous or anhydrous ferric oxide. 



Lithologic Characters.' — -The sandstones may be described in few words. 

 They are chiefly siliceous. Rarel\ T , grains of other material than quartz 

 are seen save at the bottom or the top of a bed. Within the beds them- 

 selves there is seldom any coherence. At the edges of the bluffs, where 

 the carbonates have trickled down from above and cemented the grains, 

 there is developed a tolerably firm rock, which has some economic 

 value. Considerable coloring matter, particularly ferric oxide, is locally 

 introduced. This is often the case in the Minnesota river valley, as at 

 Ottawa, Lesueur, etc. In places spherical lumps and even huge botry- 

 oidal masses are formed in the upper sand layers by the trickling down ' 

 of the carbonated waters. At Lanesboro and thence to Hokah, especially 

 in the Jordan layer, these cannon-ball-like lumps weather out in pro- 

 fusion. When broken the fragments tend to assume a rhombohedral 

 form through the cleavage of the calcite constituting the matrix. By 

 breaking these spherical masses, surfaces several inches across can fre- 

 quently be secured which exhibit in a beautiful manner the cleavage 

 planes of* calcite as they are held to the light. This is a very striking 

 illustration of the strength and persistence of that crystallizing force 

 which rebuilds broken crystals of the alums, vitriols, etc, for the chemist, 

 enlarges the quartz fragments throughout whole beds of quartzite, ex- 

 tends hornblendes and augites in fragmental and eruptive rocks, t and 

 produces the ophitic structure peculiar to many diabases. A kaolinic 

 material appears in other places to lie interstitial witli the grains of 

 quartz, precisely as in the Saint Peter sandstone above. 



*Cf. N. II. Winchell : Geology of Mil aota, Pinal Report, vol. i. 1884, p. 253. 



to. B. Van Hiee, Enlargement of Hornblendes and Augite9 in Fragmental and Eruptive Rocks : 

 Amer. Journ. Sci., 3d ser., vol. xxxiii, 1887, p. 386. 



