mid Miner ahgy. 
42. Explanation and proof. — The series of crystallisations 
belonging to common iron-pyrites is tessular. If, according to the 
method just explainedj we collect the varieties of a species, we 
shall meet with numerous individuals, which almost entirely 
agree with common iron-pyrites in all their characters,- — ^in co- 
lour, hardness, specific gravity, streak, and so on ; ' but whose 
crystalline forms belong to the prismatic system. It is these va- 
rieties that Werner had named radiated, hepatic and cristated 
iron-pyrites. Individuals whose forms belong to two diiferent sys- 
tems of crystallisations cannot be united in the same species. 
Radiated hepatic and cristated iron-pyrites, therefore, constitute 
a distinct Species. Yet this species is so similar to that of com- 
mon iron-pyrites, that we must either unite them into one Genus\ 
or relinquish ail sort of methodical arrangement. 
43. Universality of the idea of genus-. — This idea of natural 
historical genus finds scope for its application, every where in 
the mineral kingdom. Yet the similarity on which it rests, if we 
regard it as constantly of the same degree, is not always so ob- 
vious as in these examples. Magnetic iron-ore and brown 
iron-ore belong to one genus. At first sight we can discover 
no great similarity between the varieties of these two species. 
But iron-glance and red ironstone belong also to this genus, and 
together form one' species. This explains the difficulty. Be- 
tween magnetic iron-ore and the crystallised varieties of iron- 
glance, as well as between red hematite and brown hematite, 
it will be easy to discover the similarity on which this combina- 
tion into one genus is grounded. 
44. Caution in fixing the In investigating that si- 
milarity of species, on which the distribution into genera de- 
pends, it is necessary, therefore, to pay attention to the parti- 
cular condition of the varieties, to compare simple with simple, 
compound with compound, and so on. Upon the whole, the 
distribution into genera, if it does not degenerate into a mere ar- 
bitrary division, depends much more on a judgment perfected 
by practice, than the distribution into species. 
45. Except in Werner's system^ no trace of' genus. — It is well 
known, that one meets with no vestige of a natural historical genus, 
in any mineralogical system hitherto published, excepting that 
of Werner. So completely has natural history been forgotten 
