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GEOLOGV OF THE SECOND DISTRICT. 

 12. 



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In diagram No. 12, there is an attempt to illustrate a more diffused condition of llie foreign mat- 

 ters. Crystallization appears not to have influenced the form of the foreign matters at all. We 

 find, judging from the appearances produced by weathering, that the separation has taken place 

 by the molecular attraction, or by segregation. Those hard flinty places are readily discovered 

 upon the weathered surfaces of the rock, but scarcely appear where the rock has been recently 

 broken. Both the fine and coarse rocks contain siliceous masses, which are quite injurious, 

 for lime, for marble, or any of the puqjoses for which limestone is so commonly employed. 

 Although they are not seen without more than ordinary attention, yet it is not difficult to detect 

 them by their hardness. Hence, when this limestone is quarried for making lime, it is neces- 

 sary to examine the masses, and reject all those which are found hard, strike fire with steel, 

 or are difficult to break ; for all such pieces are incapable of being converted into pure lime. 



The phenomena which it has been my object to illustrate, have been thus far drawn from 

 limestone when associated with gi'anite, or with a rock confessedly unstralified. If the subject 

 of its origin were left here, there would remain still the question, whether those appearances 

 which indicate its igneous origin were not due to its association with granite, and whether the 

 non-appearance of planes of stratification is not the effect of the granite upon it ? I have, 

 therefore, examined this rock with gi-eat care, wherever I have found it associated with the 

 schistose rocks, as gneiss and hornblende, or sienite. 



Most of the following diagrams are selected from those localities where gneiss takes the 

 place of granite ; and it is proper to observe, that all which have been given, have been taken 

 with care and fidelity. It has not been my object to select only those which favored my own 

 views of the origin of this rock. I have, however, this to say, that I have seen none which 

 were equivocal in their meaning, or which at all favored the metamorphic theory. 



The diagram in the margin shows the position of a iiiass of 



limestone in gneiss, near Whitehall in Washington county. It is 



analogous to what has already been exhibited in the preceding 



4 figures, when the same rock is associated with granite. 



,^(ii,:,, .** f r.g^:^- rpj-jjg niay be taken in connection with the succeeding No. 14, 



Linieslonc. fr. Gneiss. 



