12 NEW YORK STATE MUSEUM 



Thin sections have been studied ranging from rich ore, through 

 leaner varieties, to the country rock nearly or quite free from the 

 sulphides. As would be gathered from casual examination, the 

 mineral composition of the ore is relatively simple and the 

 microscope adds little to the list of minerals already given. 

 Mention may be made of a peculiar mineral of vermicular habit, 

 which occurs in small quantities and conforms, except in its ple- 

 ochroism, to Termier's ^ leverrierite, classed by Dana as a variety 

 of muscovite. 



The country rock is a crystalline aggregate of calcite with varying 

 amounts of silicates such as diopside, tremolite, phlogopite, talc, 

 serpentine and alteration products of doubtful nature. Of these 

 minerals, calcite is, as a rule, the dominant one and forms the main 

 mass through which the others are scattered. These latter may, 

 however, so increase as to predominate, while again they may almost 

 wholly disappear. 



So far as calcite, diopside, tremolite and phlogopite are concerned, 

 no marked order of formation is evident and it would appear that 

 they must have crystallized nearly simultaneously as products of one 

 set of conditions. It is necessary, however, to recognize the fact 

 that the calcite, besides having a distinctive behavior to be discussed 

 later, is a mere recrystallization of the original calcite of the lime- 

 stone, while the silicates did not exist before the establishment of 

 the conditions under which this recrystallization took place. Some 

 idea as to what these conditions were may be derived from the 

 study of the general geology of the region. 



It has already been seen that the country rock of the zinc ores 

 is a crystalline limestone of sedimentary origin, having been 

 deposited either as a chemical precipitate or an accumulation of 

 the calcareous shells of marine organisms. In its original state, it 

 doubtless resembled many limestones of the Paleozoic or later form- 

 ations, while its presenc highly crystalline condition is due to 

 changes effected by metamorphism. This has resulted in a complete 

 recrystallization of the rock, with the production of new structures 

 and new minerals, so that, doubtless, the limestone as it now exists 

 is quite different in appearance and in mineral composition from 

 what it was before the metamorphism. 



^ Termier, P., Note sur la Leverrierite; Bill. Soc. Fr. Min., 1890, 13:325-30, 

 and Sur la Composition Chimiqiie et les Proprietes optiques de la Lever- 

 rierite, ibid, 1899, 22:27-31. 



