184 HALL AND SARDESON — THE MAGNESIAN SERIES. 



and Oneota formations. Nodule-building in all its stages can be fol- 

 lowed in the examination of these layers. 



THE DOLOMITES. 



Lithologic Characters. — Both lithologically and genetically these are the 

 most important Paleozoic rocks in the northwestern states. It is a ques- 

 tion of what they have been as well as what they are. Their lithologic 

 characters were briefly discussed before this Society one year and a half 

 ago,^ and but little of that discussion need be repeated here. All sedi- 

 mentation bands are obliterated save in rarely protected localities. A 

 pseudo-lamination is sometimes seen, produced generally by the alterna- 

 tion of finely textured, compact bands with coarser and porous ones- 

 These porous bands at times become highly vesicular and by the disap- 

 pearance of the walls of the vesicles a truly cavernous condition ma}^ 

 obtain as at Nininger and Hastings, Minnesota. When the spaces thus 

 formed become filled with white calcite a series of alternating dolomitic 

 and calcitic laminae is developed, thus producing a sort of cryptozoonic 

 structure. This structure at Lanesboro can be traced along the face of 

 the bluff for hundreds of feet horizontally and tens of feet vertically. 

 The calcite bands are usually less than half an inch broad. The nar- 

 rower ones show an interesting embedding of dolomite crystals in the 

 coarsely crystalline calcite. The mass of the rock is made up of dolomite 

 crystals and grains. The rhombohedral form is persistent in them. This 

 same tendency to develop rhombohedral crystals has been noted else- 

 where. Fischer-Benzon,t a quarter of a century ago, described the clearly 

 rhombohedral attitude of the dolomitic constituent in a series of Silurian 

 rocks examined by him. He also called attention to the various im- 

 purities characterizing those rocks. Near the Stockton quarries the de- 

 generation of the porous condition is so complete that shovelfuls of loose 

 sand consisting of dolomite rhombohedra can be taken up. This corre- 

 sponds to that very complete stage of physical alteration seen in granitic 

 districts where masses of the rock are subjected to aerial degradation 

 until only fragments of feldspar and quartz lie mingled in a bed of loose, 

 unrolled debris. 



The strongest distinction between the coarser and the finer and firmer 

 portions of these dolomites lies in the size of the grains and the absence 

 of well developed rhombohedra in the finely crystalline portions. This 

 obliteration of rhombohedra is effected through the contact of neighbor- 

 ing individuals. Impurities abound, yet they can be differentiated by 

 chemical analysis with more precision than by microscopic identification. 



*This Bulletin, vol. 3, pp. 346-348, 



fNeues Jahrbuch far Mineralogie, u. s. w., 1869, p. 853. 



