Review of Fecent Geological Literature. 345 
oxidation in all forms of pyrite is essentially the same, resulting in the 
initial production of ferrous sulphate and free sulphuric acid. Bj' the re- 
moval, decomposition, or neutralization of the latter, oxidation of the 
ferrous salt is promoted, which may be then rapidly converted into one or 
more ferric sulphates, when freely exposed to the air. In the purer 
forms of pyrite subject to but slow oxidation, the results of decomposition 
are washed away as fast as they form, and the surface of the mineral re- 
tains its brightness, if the grain lies exposed to the air on the surface of a 
stone; if buried beneath the surface the ferrous sulphate is likely to be 
immediately converted into a comparatively insoluble ferric sulphate; and 
from that the ferric oxide may be immediately deposited; if the matrix is 
calcareous or magnesian, the ferric oxide is deposited at once in place by 
precipitation, and a hepatic pseudomorph finally results. This material 
may consequently detain small quantities of lime and magnesia within its 
pores. 
Some general conclusions in regard to variation in decomposition, as de- 
rived by Prof. Julien, are as follows: (1) There is a constant association 
of pyrite with marcasi*e in intercrystallization and replacement, in the 
most intimate forms. Wherever the least deviation is noticed from the 
ordinary physical properties of either mineral the presence of the other 
may be at once suspected. (2) In regard to marcasite the tendency to 
decomposition and its absence are plainly associated with other physical 
properties. When the mineral occurs in the rarer condition of compact, 
well-formed crystals, with brilliant lustre, greyish white color, and high 
specific gravity, it resists decomposition as effectively as the stable form 
of pyrite and as most silicates. But when it occurs in crystals with lower 
lustre and density, whose color inclines to a greenish tint, or in finely 
granular, scaly, or columnar masses of lower lustre, densit}% and purity of 
color, and in which a little clay and moisture can be detected by analysis, 
such varieties are certainly inclined to more or less rapid decomposition 
by efflorescence. (3) The crystals and nodules which are generally as- 
signed to pyrite on account of tlie crystalline forms of that mineral (cubes, 
octahedrons, &c.) which the individual grains present, seldom consist cer- 
tainly of that mineral in a pure condition, save when they also exhibit its 
characteristic brass-yellow color, high density and luster, conchoidal 
fracture, and strong resistance to decomposition. When inclined to altera- 
tion, however, they invariably exhibit either a paler yellow, whitish or 
greenish color, a low density and lustre, uneven fracture and tendency to 
granular or fibrous texture. (4) Correlating then these three facts, the 
peculiar physical properties of marcasite, its common intermixture with 
pyrite, and the presence of its other physical properties in most of the 
varieties of pyrite which show ready alteration, the following conclusion 
seems inevitable : All specimens of pyrite in active decomposition are not 
pure, but are intimate mixtures of marcasite and pyrite, probably in the 
7nost minute, i. e., molecular conditions of these minerals. 
The greater tendency to decomposition witnessed in marcasite as com- 
pared with pyrite is believed to be due to a more open physical structure, 
being an interlacing network of spearhead crysUils, needles, twins, etc., 
