and their Pseudomorphs. 



157 



formation apparent to account for these differences of habit, 

 their cause or causes being entirely a matter of speculation. 



The ordinary basal cleavage of the species is well shown, and 

 the magnetic character is marked, all specimens tested being 

 strongly attracted by a rather weak magnet. 



The rock in which the pyrrhotite occurs is a very calcareous 

 sandstone associated with the upper (" fossil") ore bed of the 

 Clinton, and the mineral is found in small secondary cavities, 

 of irregular shape, lined with quartz and some calcite and 

 dolomite. The order of crystallization appears to be dolomite, 

 quartz, calcite and pyrrhotite. As stated above, celestite and 

 strontianite are present in small amounts, as is also a pale green 

 chloritic mineral, whose quantity is too small to admit of a 

 careful study. 



The placing of the pyrrhotite with reference to the other 

 minerals present affords a very pretty instance of selective pre- 

 cipitation which is particularly marked in the case of the 

 minute hexagonal plates. When a cavity contains calcite, the 

 pyrrhotite is deposited upon this mineral almost exclusively. 

 Calcite, which to the naked eye looks dusty, is shown by the 

 microscope to be sprinkled over with beautiful hexagonal plates 

 of pyrrhotite, and the latter often occupy depressions in the cal- 

 cite, looking as though they had been pressed down into it 

 when it was in a plastic condition, although, of course, such is 

 not the case, the phenomenon being due to solution. 



When calcite is lacking in a cavity, dolomite supports the 

 pyrrhotite, and it is only when the crystals of the latter become 

 large or very abundant that they rest upon quartz. Whether 

 the influence of the carbonates is chemical, physical, or both, 

 is not entirely clear, but the frequent occurrence of pyrrhotite 

 plates in cavities of the calcite and dolomite is strongly sug- 

 gestive of chemical action. It is worthy of note that no pri- 

 mary pyrite was found associated with the pyrrhotite, although 

 the former mineral is rather frequent in the Clinton rocks, and 

 at a quarry only half-a-mile distant, and in the same horizon, 

 occurs in good crystals up to an inch in diameter. Seemingly 

 the iron sulphide usually precipitated as pyrite in the Clinton 

 rocks was, by some peculiar condition, in the particular locality 

 under consideration, crystallized as pyrrhotite. The associated 

 minerals show no corresponding peculiarities to indicate the 

 causes leading to the formation of pyrrhotite, and as in their 

 conditions of original deposition, as well as in subsequent 

 history, these rocks must be practically identical with the rocks 

 of adjacent quarries, the modifications needful to produce pyrr- 

 hotite instead of pyrite must have been so slight as to leave no 

 means of tracing them. This is not, perhaps, surprising in 

 view of the close similarity in the chemical composition of the 

 two minerals and yet it is a fact that, in spite of this similarity, 



