128 Peckham and Hall—Thomsonite from Minnesota. 
a seam, instead of being collected around a nucleus as a spheero- 
lite. They show parallel green fibers meeting along a median 
suture and correspond in their manner of occurrence to Zirkel’s 
description of axiolites in the rhyolites of the 40th parallel.* 
The amygdules of the green variety rarely exceed in size a 
small hickory nut. As before stated, they are not generally 
found intermingled in the rock with the other forms, but have 
special localities—they filling nearly all the amygdaloidal cavi- 
ties within a given limit, whose boundary at the same time is 
not sharply defined, F requently the forms of Number I or IL 
are enveloped in a green covering of considerable thickness. 
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as altered forms of Number III, as the condition of the iron 
might indicate. No amygdule has come under our observation 
which exhibited a nucleus of plaeed o surrounded by Num- 
ber I or II. On the contrary, e quite a number in 
which, through a thin translucent shell of Number III, the pink 
interior can be discerned. And we also have fragments, and 
amygdules have been cut, which show the external crust of 
Number III passing toward the center into the radiated form of 
Number I 
In determining ne oxygen ratio for Number II, the silica 
appeared to be too high. We had previously suspected the 
presence of free hea from the exceptional hardness of all of 
these varieties. As the microscope showed the ferric oxide in 
every case to be free, we concluded to compute the percentages 
for Number II to 40°45 per cent of silica, the amount found in 
Number I, and exclude the iron oxides. We were much sur- 
prised at the results, which are given below: 
I. 
Il. II 
SiO 40°45 40°45 40°605 
Al,O, 29°50 29°37 30215 
a 10°75 10°43 10°37 
K,O 0-36 0°42 0°49 
Na,O 4°76 4-28 4-05 
H,O 13-93 13-93 13°75 
99°75 98:88 99°48 
Fe,0, 0°23 088 FeO -40 
99°98 pr.ct. 99°76 pr.ct. 99°88 pr. ct. 
* U. 8. Geol. Explor. 40th Parallel, vol. vi, p. 166 et seq. 
