GEOLOGICAL SOCIETY OF PENNSYLVANIA. 



319 



quartz lodes, there is every reasonable probability that 

 they descend proportionately beneath tlie surface, and 

 that they will be found to continue to the lowest depth 

 accessible to the practical miner, and therefore are only 

 limited by the circumscribed power attending all human 

 operations. Whether the metalliferous properties of 

 these veins be permanent or modified, at increased 

 depths, is a secret which time and actual proof can alone 

 determine. 



Harper's Ferry Section. PL 17, fig. 1. 



It is unnecessary to make other than a brief reference 

 in this place to the section which comprises a large por- 

 tion of the country between Winchester and Baltimore, 

 because ail its details will be found inserted in that dia- 

 gram, and a repetition would be superfluous. The west- 

 ern extremity of this line passes over 30 miles of blue 

 limestone, and occasional beds of hydraulic limestone, 

 which have been previously adverted to in Mr Clemson's 

 paper. These latter rocks are crossed in the vicinity of 

 Charlestown, dipping nearly at 45° in some positions. 

 They are here quarried ; and, after calcination, yield a 

 hydraulic cement of considerable repute. Two varie- 

 ties of this rock, together with the common blue transi- 

 tion limestone of this valley, have been analyzed by the 

 above named gentleman, and the result is contained in 

 the following note, extracted from the Journal of the 

 Franklin Institute.* 



* Analysis of Hydraulic and Blue Limestone^ from Jefferson County, Vir- 

 ginia. 



I analyzed two varieties of this limestone : the one was of a black colour, 

 had a conchoidal fracture, and was susceptible of a fine polish, differing- from 

 the grey variety, inasmuch as the latter contains a greater proportion of alu- 

 mina, giving the hydraulic properties, and is free from bituminous matter, 

 the evident cause of the black colour observed in the first. These two sub- 

 stances being found in immediate contact with the blue limestone, I add an 

 analysis of this rock from the same locality : — 



