40 ON RYRITE AND MARCASITE. [bull. 186. 
form of pyrite but supposed to consist very largely of marcasite. 
The four specimens, Nos. 29, 30, 31, and 32, consist of characteristic 
pyrite crystals, and it* the oxidation method be admissible they are in 
fact free from marcasite, yet they all show abnormally low densities. 
The only conclusion which can he drawn from this is that the density 
alone does not afford a reliable means of detecting the presence of mar- 
casite in pyrite unless the absence of all other contaminating substances 
be first conclusively proved, and that isometric crystallization in itself 
affords a proof, almost amounting to certainty, that the sulphide is 
actual^ pyrite. It is undoubtedly true that perfect pyrite crystals 
may inclose marcasite, a good case of which is the Crow Branch speci- 
men, No. 23, but such cases are comparatively rare, and the inclosure 
can usually be detected by breaking the specimen and carefully exam- 
ining the color. In case of doubt a determination of the density is prac- 
tically of no value, and resort must be had to the oxidation method. I 
have examined some of the marcasite specimens from the same collec- 
tion, as well as others which are supposed to contain pyrite, and in gen- 
eral the latter can be detected by its color after careful cleaning with 
acid. I have shown that certain specimens of marcasite do contain 
pyrite inclosed in such small masses as not to be capable of detection 
in this way (Nos. 22 and 23). Small, warty excrescences on marcasite 
crystals are frequently pyrite, and when the conditions controlling 
the formation of the two sulphides are practically in equilibrium these 
are likely not only to form but to be completely overgrown and 
inclosed in the crystal of marcasite. A marcasite which occurs mixed 
with visible masses of pyrite is therefore likely also to contain such 
inclosures which can not be seen and whose presence can be detected 
only by the indirect oxidation method. That both pyrite and mar- 
casite with abnormal density are essentially prone to decomposition 
is doubtless true, and while in certain cases this may be traced to an 
admixture of the two, in others it is due to inclosures of another 
nature. The lack of homogeneity not only permits the existence of 
fissures admitting air and moisture, but the formation of these is in 
such cases promoted by unequal expansion and contraction. 
Fibrous marcasite is so often referred to that it may well exist, but 
so far as my somewhat limited range of examinations goes, a fibrous 
structure affords absolutely no presumption in favor of a specimen 
being marcasite; on the contrary, fibrous pyrite appears to be much 
more common than fibrous marcasite, the latter in concretions 
tending rather to assume the coarsely columnar structure. 
XI. PARAMORPHISM. 
The literature of the paramorphism of pyrite and marcasite is 
exceedingly scanty, and such as there is lends no support to the 
view that true transformation paramorphs exist. Those described 
are apparently either replacement or incrustation pseudomorphs, the 
