MINERALOGY AND PETROLOGY. 961 



Glauconite. Sphene.] 



may be true that they result from the same sort of alteration as mentioned under 

 biotite, but in the process of formation took also water into crystallization. Their 

 alkaline base varies much, embracing also sometimes a notable amount of protoxide 

 of iron. 



As the microscopic characters of muscovite and biotite are not always distinct- 

 ive, so the chlorites fade also into biotite. Chlorite sometimes replaces muscovite 

 or biotite (Nos. 2265, 2277), one lamella after another, in whole or in part, in the 

 same way that biotite is intimately associated with muscovite. Indeed, there seems 

 to be an easy gradation from one end of the series of these foliated secondary min- 

 erals to the other, i. e., from muscovite to chlorite, the specific names depending on 

 the varying amounts of the bases present and ready to enter into combination at the 

 moment of crystallization. As water and magnesia increase, the mineral thalite (No. 

 91B) seems to represent the extreme of the series over against muscovite. 



Glauconite. The greensand of the Taconic iron ore was fully described and 

 discussed by Mr. J. E. Spurr, in Bulletin x of the Minnesota Survey. He showed the 

 various microscopic changes that transpired in that substance which resulted in the 

 production of the ores of the Mesabi Iron range. The acumen with which he ferreted 

 out this as the primordial element in the taconyte and showed that both the silica 

 and the oxide of iron resulted from the transition from unstable chemical compo- 

 sition to silica and hematite, the most stable condition of those elements, is worthy 

 of all admiration, and his process and his result will not here be called in question. 

 He presumed, as an ulterior source of the greensand, that it may have been of organic 

 origin, and perhaps depended on Foraminifera, and the writer has given in this 

 volume (page 366) further facts that tend, with a little idealization, to indicate the 

 foraminiferal origin of this greensand. But the greensand was not probably glau- 

 conite of foraminiferal origin. According to descriptions and illustrations contained 

 in Part II (Nos. 1276, 1294, 1530, 1630A, 2052, 2138), it is rendered highly probable 

 that this greensand was a more or less devitrified volcanic glass sand. The full 

 discussion of the origin of the taconyte of the Mesabi Iron range is given under that 

 head in the subchapter devoted to petrology. 



Sphene. This mineral, when in the metamorphic rocks, is plainly the result of 

 secondary forces, and has resulted from the.presence of titanium in older, or original 

 minerals, usually ilmenite or titaniferous magnetite. Such titaniferous minerals 

 result from the disintegration of the original basic rocks, i. e., the greenstones, and 

 have permeated, in greater or less quantity, all the later rocks. That this mineral 

 is, in this sense, secondary also in the igneous rocks in which it occurs, is indicated 

 by its absence in the original rocks, and by its morphologic relations to the minerals 

 with which it is associated. To a large degree sphene has a powerful crystallizing 



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