wiluams.] WASHINGTON TO CUMBERLAND. 275 



of here. The rock near the station is a nearly vertical feldspathio 

 gneiss, but just beyond the sericite schists are seen. 



This place is directly north of the Great Palls of the Potomac, the 

 source of Washington's water supply, near which the gold mines of 

 Montgomery County are situated. 



Derwood is where the schists are quite vertical, and where the axis 

 of the Piedmont tan crosses the road (see section Pig. 1). 



Gaithersburg. Here are several lenses of serpentine in the schists; 

 the hitter are fully exposed in the deep ravine of (ireat Seneca Creek. 



Boyd. Here a large mass of intrusive trap occurs, which is petro- 

 graphically identical with the Triassic dike of diabase just west of it. 

 The age of both is in all probability the same. 



Dickerson. The isolated mass of Sugarloaf Mountain, which has for 

 some time been visible, is now well seen on the right (north). It is an 

 east dipping monocline of sandstone, divided into two parts by a lon- 

 gitudinal \ alley, and is probably due to a double thrust of the sandstone 

 from the east. Near this point, also, a broad transgression of Triassic 

 red sandstone is entered, and a little beyond a great dike of diabase 

 is cut through. This latter traverses the whole State of .Maryland 

 from north to smith, and passes into Virginia. Its hardness makes it 

 a topographical feature, which is known in the neighborhood as "Iron- 

 stone Ridge." A mile or so farther, the Monocacy River is crossed by 

 a high bridge; a short break in the red sandstone exposes the shales 

 and blue limestone of Frederick Valley (Hudson and Trenton), which 

 at the railroad are mostly covered by alluvium deposited by the Poto- 

 mac River, Which is here for the fust time approached. 



At Tuscarora red sandstone appears again and continues to Wash- 

 ington Junction. For a mile or more before reaching the latter station 

 the upper member of the Trias is well exposed along the line of the 

 railroad. This is a coarse conglomerate of rounded, or sometimes angu- 

 lar and broken, pebbles of variegated Paleozoic limestone, embedded 

 in a red argillaceous matrix. It is a striking rock and enjoys qnite a 

 wide celebrity as a decorative stone. If is known as ''Potomac Mar- 

 ble," or " Calico Rock," and is the material from which tbe great col- 

 umns in the rotunda of the Capitol at Washington were made. 



At this point (Washington Junction) connection is made for the city 

 of Frederick, some L5 miles to the north, which occupies the center 



of the broad, fertile Limestone valley. Before reaching this place, how- 

 ever, the main line of the Baltimore and Ohio railroad diverges and 



pursues a course down the Patapsco River direct to Baltimore. Near 

 this point of divergence, on the Monocacy River, 12 miles NK. of Wash- 

 ington Junction, an important battle of the war was fought in IStll. 

 and the northern extension of the same valley in Pennsylvania was the 

 scene of the (?reat battle of Gettysburg. 



