PSEUDOMORPHOUS MINERALS OF NEW YORK. 249 



modify each other's form ; as for example, when carbonate of 

 lime and carbonate of magnesia are found mixed together, as in 

 dolomite, the angle of the rhomb is intermediate between that 

 of the two simple carbonates. These, and many other interest- 

 ing facts, have been well explained by the recent investigations 

 of the subject of isomorphism. But in order to account for the 

 facts just adverted to, it is necessary that we should admit that 

 foreign substances, not known to be isomorphous, may be intro- 

 duced either dm-ing the process of crystalhzation or subsequently 

 to its completion, without materially changing the form of the 

 crystal. Do not the facts which I have aheady presented, be- 

 sides many others known to chemists and mineralogists, waiTant 

 this conclusion ? And if they do, we have at least taken one 

 step towards a theory of these changes. 



There are one or two examples in confirmation of the above 

 views, too striking to be omitted. It is well known that many of 

 the beautiful and perfect crystals of quartz found at Little Falls 

 and Middleville, in Herkimer county, have masses of anthracite 

 diffused through them. In some instances, indeed, they are so 

 copiously studded with minute particles of this substance as to 

 give the crystals a black color. I have also observed that the 

 cavities in the calciferous rocks which contain these crystals are 

 lined with the same black powder. 



The dodecahedral crystals of quartz, which are associated 

 with iron ore, in St. Lawrence county, often have a porous or 

 spongy texture, with a considerable mixture of oxide of iron, 

 while their form remains unaltered. 



Another instance of a similar kind will be found in some 

 foliated or semi-crystalline specimens of Kyanite, in which thin 

 plates of mica are constantly interposed, between the folia of the 

 mineral, without having in the least interfered with its crystalline 

 arrangement. Now we have only to extend these facts to afford 

 an explanation of the phenomena observed in the hornblendic 

 and pyi-oxenic minerals which have been described. Thus the 

 hornblende, the spinelle or the pyroxene, may retain its crystal- 

 line form, perhaps in a skeleton state, while foreign substances, 

 as alumina, or serpentine, or steatite, are intruded. 

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