340 The American Geologist. sane 
feldspar, scapolite, etc. In amphibole, alumina varies from 
o to 18.20 per cent.; iron, from o to 40.40 per cent.; lime, 
fromy. 0, to 20:47 pen cent.;5\ etc. 
Now the physical characters of our mineral are those of 
glauconite. Professor Wolff* described it as of “a brownish- 
green to clear-green color, partly isotropic, partly aggregate 
polarizing, in feebly polarizing dots and specks. Hardly any 
pleochroism, no cleavage. They resemble in all physical char 
acters glauconite-grains.” + 
One of the most characteristic features of the Mesabi 
green silicate as described and figured by the writerz, is the 
peculiar process of decomposition, marked by the appearance 
in the thin section of numberless tiny silica rings, which 
erow until they unite, the iron which is at the same time 
separated giving to the residual glauconite a progressively 
~darker and browner color. 
Zirkel remarks§ that in glauconite sandstores (green sand- 
stone) : 
“The glauconite-grains, which are sometimes sparse, sometimes 
abundant, in this sandstone, break up on weathering into many concent- 
ric spherical shells,|| and the iron oxide of the glauconite changes 
into hydrated oxide of iron, whereby the greenish color of the stone 
gradually is changed to a light brown.” 
Since his Minnesota work the writer has found in Alaska 
ancient glauconite-bearing rocks (perhaps Silurian) where the 
glauconite shows the same process of decomposition and trans- 
formation. He quotes from himself :{ 
“The jasperoids are rocks consisting chiefly of silica, generally 
chaleedonic or cryptocrystalline, and are plainly derived from the sil- 
iicatlonNon ober LOCKS. sl sean 
“An interesting and important variety is the glauconite jasperoid 
or taconyte, such as has been described by the writer as occurring 
abundantly in the iron-bearing rocks of the Mesabi range in Minne- 
sota. In the rocks of the Rampart series° it also appears that the 
glauconitic limestones pass into jasperoids, which are colored red, 
green, brown, or gray by iron in its different forms, or occasionally 
by manganese, or which are light gray or nearly white, as a result 
Ol, thessepanation 1Of the mmoles sn en nn eee 
* Bull. X, Minn. Geol. and Nat. Hist. Survey, p. 331. 
+ The italics are mine. (J. E. S.) 
t+ Ibid., p. 133, and plate VI, figs. 1 and 2. 
§ Lehrbuch der Petrographie, vol. iii, p. 728. 
|| Whose cross-sections are rings. (J. E. S.) 
{ 18th Ann. Rep., U. S. Geol. Surv., part iii, pp. 160, 161, 164, 165, 166 
° Alaska, Yukon river. 
= wry 
