COMPOSITION OF THE DOLOMITES. 185 



Silicioiis Scfjrcyntions. — An oiUitic structure is locally present, and the 

 oolite is both silicious and doloniitic. The former is likely to be found 

 in those l)eds which carry the silicious concretions and silicified fossils, 

 mentioned on page 347 of the former i)aper just referred to. These con- 

 cretions present many interesting problems. Tliey consist of silica in 

 two different })hases: (1) Quartz attached to original grains in crystallo- 

 gra]»hic continuity and (2) microcrystalline, tliat is, chalcedonic quartz. 



With each of tliese jihases are many original and well rounded grains 

 of (piartz wliicli, with the chalcedonic as well as the crystallized phase, 

 serve as nuclear grains for the segregation of considerable silica. 



The second type was used to a considerable extent l^y the former 

 dwellers along the river valleys and lake shores in making arrow-heads, 

 spear-iH)ints, etcetera, because the segregations were freiiuently large 

 enough to enal)le the workmen to chip down from fragments until a 

 well dressed tool or weapon was [)erfected. 



Segregations of silicious sand grains are frequently seen in vertical 

 arrangement, as if canals had l)een dissolved in the dolomite and tliese 

 grains from subsequent sediments had fallen down to occupy the cavi- 

 ties thus formed in the rock. Such s})ecimens were i)articularly fine 

 about Merriam Junction and Isinours, Minnesota. 



Glauconite Grains. — In addition to the miscellaneous impurities just 

 mentioned, including the several phases of free silica, there occurs 

 quite generally in certain positions a green mineral wdiich is known as 

 glauconite. Some years ago Professor Peckham analyzed the mineral. '=^ 

 The mean of four analyses is as follows : 



,SiO., 48. 18 per cent. 



Fed 27.08 



AlA <5.97 



K^O 7.40 



NiU) 1.25 



U,ij 8.75 



T<.tjii 9y.()3 



This result agrees closely with the composition of the New Jersey glau- 

 conites t of Lower Tertiary and Cretaceous age and that of recent dei)osi- 

 tion as determined by v. Giimbel J in liis examination of the dredgings 

 ])rought uj) by the Gazelle. Still, taking tlie results in reversed order 

 from tliat cited above, we note in following down from recent glauconite 



• 8. P. Peckham : Report on Chemistry, Ann. Rep. Oeol. and Nat. Hist. Survey, Minnesota, 1876, 

 p, BI : 1879, p. IW. 

 tDiinft's Sy>*t«MU of .MinoraloKy, tiftli edition, 1H82, pp. U>'2, 403, 

 : N.iu-s .Iiihrbiicli fur Mineralogie. u. s. w., Kef. UH'J, i, p. M. 



