186 HALL AND SARDESON — THE MAGNESIAN SERIES. 



to the Cambrian a steadily increasing percentage of alumina. The mineral 

 usually occurs in grains more or less regular in outline, many of them 

 being elliptical or spherical and others decidedly subangular. In the 

 Saint Lawrence there are places where the glauconite is distributed in 

 the interstices of the rhombohedral grains of dolomite, showing almost 

 no tendency toward segregation into the compacted granular condition, 

 as is the case with the silica. The color is a bright green, sometimes 

 browned with ferric oxide; in either case they form a sharp contrast to 

 the dirty white, the usual color of the normal dolomite, in which they 

 are imbedded. These glauconite grains are extremely finely textured, so 

 fine indeed that it requires high magnification to analyze them. They 

 are slightly dichroic and respond in a very weak degree to tests under 

 crossed nicols. No relation could be traced between the form of the 

 grains and the form of possible forameniferal shells or other organic re- 

 mains ; neither could any indication of detrital origin be seen. It may 

 be added that it is difficult to understand, with the vast changes in bulk, 

 chemical composition and crystalline condition which these rocks have 

 certainly undergone, how any organic shells or even casts could have with- 

 stood the varied vicissitudes to which they have been subjected. How 

 the green color could be retained through all the changes, chemical and 

 physical, which the rock formation has undergone is equally difficult to 

 understand. The origin, therefore, of these glauconite grains is looked 

 for not in the conclusions of v. Giimbel,^ Murray and Renard'f or Clark,]; 

 who closely follows Murray and Renard, but rather in the chemical con- 

 ditions of the mingled mineral matters of the including rocks, a view 

 which Dr. T. Sterry Hunt repeatedly set forth. § 



Doloinitic Conglomerate. — In several connections the writers have already 

 called attention to the conglomeratic condition found in the top of the 

 dolomites. Broadly stated, it ma}^ be said that the summit of each of the 

 three dolomitic formations is to a considerable extent conglomeratic. 

 This texture is not always so clearly conspicuous as in the Shakopee at 

 Lanesboro, Minnesota, and at other places in Minnesota and Wisconsin. 

 There is great diversity in the size of the pebbles in these conglomeratic 

 layers, but a remarkable uniformity of shape exists. They are nearly 

 always lenticular. In texture they are finer than the dolomite of their 

 matrix or of the lower layers of the formation. The conglomeratic phase 

 may be retained, since the finer and denser texture of these pieces resists 



* Ibid., p. 20, 



t Challenger Expedition Reports, volume on Deep-sea Deposits, p. 387. 

 t Ann. Rep. Geol. Survey of New Jersey, 1893, p. 238. 



§Chem. and Geol. Essays, 1878, p. 303; Mineral Physiology and Physiography, 1891, pp. 190, 309; 

 Systematic Mineralogy, 1892, p. 257. 



