Theoretical Considerations. 325 



fragments were piled together, it is not difficult to conceive that the in- 

 terstices might have been filled by the iron from solution in water. 



The same difficulty occurs in the case of those extensive metallifer- 

 ous veins that have been referred to, whose gangue is quartz, en- 

 closing masses of mica slate and forming a kind of breccia. But the 

 whole subject of mineral veins has a great deal of mystery hanging 

 over it, and is probably less understood than any other department of 

 geology. 



What shall I say of the origin of the Washington buhrstone? We 

 have every reason to conclude that the French buhrstone was depos- 

 ited from water. And that at Washington differs from it chiefly in 

 being more arenaceous and tender. Probably, therefore, it had a 

 similar origin. But what shall be said of the occasional fragments 

 of feldspar and hornblende which it contains ? Certainly these give 

 it somewhat of a mechanical character, and their production and in- 

 troduction are difficult to explain. Can it be, that subsequently to its 

 deposition, it has been subject to the action of a heat so powerful that 

 a partial fusion took place, and a few imperfect crystals of feldspar 

 and hornblende were thus produced ? That granite exists in quantity 

 not far from this rock, is rendered probable by the facts already men- 

 tioned, viz. that veins of it occur at a quarry of quartz rock in the same 

 hill, and that the strata at this quarry dip a few degrees to the west, 

 while the burhstone dips about as much to the east. And this granite 

 might have furnished the requisite heat. 



The conglomerate quartz rock originated probably like other con- 

 glomerates : that is, we must first suppose an abrasion of existing 

 strata, and then a consolidation of the materials thus worn off, either 

 by heat or simple desiccation. In the present case, heat was probably 

 an important agent. Otherwise I know not how to explain the marks 

 of a crystalline structure which it exhibits ; as much, indeed, as the 

 oldest mica slate. But until this rock is found in place, it will be use- 

 less to spend much time upon its theory. 



The varieties of quartz rock associated with mica slate, talcose 

 slate, and gneiss, probably had an origin similar to that of these sev- 

 eral rocks. That they have all been acted upon powerfully by heat, 

 I cannot doubt ; but not until after their deposition. I confess myself 

 inclined to the opinion, that all these strata originally resembled our 

 present secondary strata ; and that the agency of the unstratified 

 rocks has rendered them crystalline. Thus the quartz rock thai con- 



