Lesley.] 472 [December. 



southeast side of the Grreat Valley, from the Hudson river to Ten- 

 nessee and A-labaraa, and adding what we know of similar deposits, 

 produced in a similar way, out of the exposed outcrop edges of the 

 same rocks in the limestone valleys further back towards the Alle- 

 gheny Mountains (such as Kishicoquillis, Nittany, &c.), and deposits, 

 in the same geological positions in Lancaster and Chester counties, 

 we can divide them with great certainty, as stated above, into two 

 classes, the slate-a-op banks, and the sand -lime-crop banks, the 

 former being always geologically underneath the latter, as represented 

 in Fig. 1. 



The cavernous condition of the formation which crosses the An- 

 tietam creek at Mont Alto is evinced by the numerous sink holes 

 and ponds lined with clay, and by the absence of small streams, and 

 by the curious topography of the whole slope of the South Mountain, 

 the want of any definite run to the vales, the bowl-shaped aspect of 

 every part of the surface, and the disappearance of the mountain 

 brooks on their way towards the centre of the valley. In other 

 valleys (as e. x. in Sinking Creek Valley, near Altoonu) the num- 

 ber and the extent of the caverns astonish and delight the beholder. 

 Where the dip of the rocks is steep there is not the same chance for 

 the formation of caverns ; and the depth to which the disintegration 

 of bed, in other words, the formation of ore, can go, is necessarily 

 limited. On the contrary, where the dip is gentle the dissolution is 

 extensive, and the ore abundant. 



Within the first half mile there have been excavated several large 

 pits. The bank at present wrought is 2200 feet from the furnace. 

 It is called the Home-bank, and furnishes the principal data for 

 estimating the quantities of ore in the whole belt, Fig. 2. 



The excavation is between one and two hundred feet long, and of 

 the shape shown in the figure. Its mouth is a cartway between 

 walls of surface clay or common stripping. Its head is a steep slope 

 of clay, covering ore, from 40 to 50 feet high, behind the top of 

 which rises the mountain side 50 feet higher, to a gently sloping ter- 

 race, as shown section, Fig. 3. 



As there are but from 5 to 10 feet of stripping, and the ore in 

 fact sometimes comes within that distance of the surface, the plan 

 shows at a glance the immense extent of the ore ground. The new 

 workings are ordinary gangways, timbered and lagged where needful, 

 with cross galleries driven to the right and left, in an irregular man- 

 ner, but so as to leave 50 foot pillars of ore between them, and not 

 kept carefully upon a level. In fact, one of the gangways to the 



