BRECCIATED STRUCTURE OF THE SHAKOPEE. 347 



regation of impurities a distinct tendency to an oolitic structure is fore- 

 shadowed (plate 12, figure 3, is from a slide prepared from tins Frontenac 

 dolomite). The rock from the new quarry at the same place has a more 

 compact structure, a finer texture, and a lighter color. The determi- 

 nation of purity has not been made by chemical analysis of the speci- 

 mens from these two quarries. All the compact and vesicular phases 

 that have been noted can be seen at scores of places among the many 

 exposures of these dolomites. 



The brecciated condition of these rocks and the oolitic phase, which is 

 also seen, have both been mentioned. Slides prove only the more clearly 

 what can lie ^rvn with the unaided eye in these phases. The angular frag- 

 ments which have been thrown together in the breccia show many differ- 

 ences in texture and in mineral composition; some of them have quartz 

 grains, others are very fine. The oolitic structure seems to be due to a 

 molecular or chemical readjustment of the material. But the siliceous 

 oolite shows certain points of interest in addition to those just named. 

 While many specimens have been seen from different depths in this 

 series, the most common occurrence is at the top of the upper Shakopee. 

 Large masses of microcrystalline silica are found segregated in these 

 dolomitic layers. It appears that frequently rounded grains of quartz 

 serve as nuclei around which the silica coming down from the overlying 

 sands gathers in crvstallographic continuity, building out to a consid- 

 erable size these small grains, and then becoming imbedded in a matrix 

 of microcrystalline (chalcedonic) silica (see plate 12, figure 4). These 

 masses of oolite were doubtless formed in the same way as were the 

 segregations of .silica so frequently met with, notably at Stillwater, Red- 

 wing and Winona, only here there are nuclei around which tin' silica 

 can arrange itself, while there a deposition on surfaces, within cavities, 

 ami along crevices presents a microstructure partly chalcedonic and partly 

 agatoid. Thin sections show very beautiful and intricate microgranular 

 growths. 



Chemical Composition. — In chemical composition the dolomites as a 

 group show a heavy proportion of impurities, particularly silica. When 

 these impurities alone are considered, there is seen to he considerable 

 variation in the composition of the beds; when the carbonates are con- 

 sidered, the variation from a typical dolomite, that is. a rock in which 

 CaCOj : Mg<'<> :; 1:1 54.4: 15.6, is no more than would naturally be 

 expected in a rock series underlying many thousand square miles. The 

 variation alluded to is based on the quantity of these two carbonates in 

 the nick to the exclusion of all other constituents ; MgC0 8 is not pitted 

 againsl the held, as in some instances is the case. 



X I. VI I I '.i i i.. i.e. i . Soi , \m.. Vol. :;. 1891. 



