340 BREACHES IN THE INNER 



Masses of this siliceous material, possessing different 

 shades of whiteness, are very frequent along the road, as 

 you pass through Loudon county on the south, and 

 through Montgomery and Frederick on the north side of 

 the river. 



Yet it must be observed, that the mixtures of quartz 

 are various in this region. Where the Potomac pene- 

 trates the strata of rocks at the Little Falls above George- 

 town, they assume the forms of granite, granitine, and 

 micaceous shist. In some instances, quartz and schoerl 

 are associated. In others mica and garnet are blended. 

 Frequently quartz is found by itself; and then again mica 

 is aggregated into enormous masses, forming the high 

 banks and much of the bed of the river. Several other 

 mixtures of these materials are found hereabout. The 

 micaceous shist containing small garnets may be seen 

 advantageously at the Chain Bridge, three miles above 

 Georgetown. The operations necessary for making a 

 passage over the Potomac, have exposed the strata in a 

 manner that favours examination. 



The great falls of this river, ten miles higher up the 

 country, are formed chiefly of micaceous shistus. The 

 quantities are exceedingly great, and compose the high 

 perpendicular and overhanging sides of the stream, as 

 well as its bed, and the rocky islands between its two 

 banks. The interior locks of the canal made here by 

 the Potomac Company for facilitating intercourse, are 

 dug through strata of micaceous shist. 



But the opening through the Blue Ridge at Harper's 

 ferry is of a still different character. The gap reaches 

 from its summit to its base, and exposes to the observer 

 its internal eonsitution. It is in such places that the pe- 



