S19 



&c. the basis of which is a hornblende porphyry, or 

 porphyritic hornblende. Of these the South Mountains 

 (or blue ridge) abounds in Virginia, Maryland, Penn- 

 sylvania, and, perhaps, throughout the whole extent of 

 its range. The mamillary masses which they contain, 

 are either quartz, felspar, or epidote, and in some in- 

 stances other substances. These I have examined, in 

 place, on different parts of the ridge, in Maryland and 

 Pennsylvania, with scrupulous attention; and I have 

 no reason to believe that they have suffered any more 

 by decomposition, or that they are more liable to it, 

 than any of the other rocks, in which hornblende is the 

 predominant substance. This opinion is founded on 

 the following facts which are observable in almost 

 every instance where these rocks are presented to view. 



The little irregular and isolated masses of quartz 

 and epidote, which enter into the composition of these 

 rocks, seem to pervade the whole body of their sub- 

 stance, and every surface that is exposed to view, ap- 

 pears studded with them; some of which project an 

 eighth, and some a quarter of an inch above the sub- 

 stance in which they are enclosed. 



These pebbles of epidote and quartz, not being liable 

 to decomposition by exposure, have retained nearly 

 their primitive form, whilst the basis of the rock has 

 experienced, in the course of time, a slight diminution, 

 which gives to the pebbles the projection which I have 

 mentioned. But that the rock has experienced no fur- 

 ther decomposition, is obvious from there being few or 



