98 Fealherslonhaugli^ B Geological Report. 



reaching the Monocacy river, a calcareous breccia, coming in 

 from the northeast, the same of which the columns in the 

 legislative halls of the Capitol are made, appears in the ridge, 

 alternating with the red shale in broad seams, and in many 

 places mixed up with it. It is quite apparent that the breccia 

 and the shale have been contemporaneously deposited. Thirty- 

 eight miles from Georgetown the ridge is about eighty feet 

 higher than the canal, and still dips east, but the breccia soon 

 discontinues, and the red shale presents a more horizontal 

 appearance, when the ridge ceases, and a small valley occurs, 

 until, at {oviy miles, the country rises into a ridge again of red 

 shale and sandstone, still with a southeast dip. At forty-six 

 miles and a half, the ridge is distant about two hundred yards 

 from the canal, but shows a good section dipping to the west. 

 At forty-seven miles the breccia comes in again in broad 

 seams, dipping to the west, and unmixed w^ith any other rock, 

 although the pebbles are in many instances set in the red shale, 

 xls this breccia is one of those geological phenomena which 

 explains in a most satisfactory manner the nature of the causes 

 which have in ancient times modified this portion of the sur- 

 face of the earth, I shall revert to it after pursuing the line of 

 the river somewhat further to the northwest. 



At the Point of Rocks, forty-eight miles from Georgetown, 

 the Potomac issues from the Cotoctin mountains, which form 

 the eastern flank of the Atlantic primary chain. This chain, 

 mineralogically considered, is a mass of primary slates, sand- 

 stones, and quartz, having a northnortheast direction, and 

 running, with a breadth of about fifteen miles from its western 

 to its eastern flank, through an extensive area of limestone. 

 Geographically considered, it consists of tv/o ranges of hills, 

 divided by the Middietown valley, the westernmost of which, 

 in this neighborhood, is called the Blue ridge, and the eastern 

 one the Cotoctin mountains. On arriving at these last, a re- 

 markable change takes place in the aspect of the country ; 

 mountainous masses, forn:-ed of many varieties of primary slates, 



