5 



matter. The yellow variety owes its colour to the presence of 

 a small portion of the oxide of iron ; it is usnally compact, oc- 

 casionally crystalline, and passes into marble. Veins of milky 

 quartz are contained, and detached masses are seen scattered 

 over the surface. This rock constitutes a portion of Monahan, 

 Hellam, East Manchester, York, Paradise, Codorus, Heidle- 

 berg, and Maeheim townships, forming a band which runs 

 through the county from south-west to north-east, dividing the 

 same into two pretty equal portions. In some of the adjoining 

 states seams of limestone in concordant stratification are quar- 

 ried and burnt for hydraulic cement, with the composition of 

 which you have already been made acquainted.* The limestone 

 of York county is burnt into lime, which is used for building, it 

 contains no magnesia, and is employed for the uses of agricul- 

 ture. This rock alternates distinctly with the schistose rocks of 

 the transition series, and frequently passes insensibly into shales, 

 without any apparent line of demarcation betwixt the two rocks. 

 When the limestone becomes schistose it is called slate limestone ; 

 in this case there is always an insensible mixture with a differ- 

 ent rock, the stratification being either straight or contorted. 



In this formation there exists a cavern of considerable im- 

 portance, situate at a short distance a little to the north of 

 York; a branch flows in on one side, and reappears on the 

 other side of the hill. There is also an opening into it from the 

 top of the hill : a stone thrown in at this spot may be heard 

 rumbling for a considerable time. The aperture below is too 

 small for ingress, and owing to the precipitate descent from 

 above, the only possible mode of gaining entrance would be by 

 means of a rope from the summit. 



FELSPATHIC AND AMPHIBOLIC ROCKS. 



As you leave the limestone of Monahan township, and ascend 

 those elevations known as the Round Top hills, the rocks change 

 and display an evident entritic character. The base of the rock* 

 when sufficiently porphyritic as to distinguish the imbedded 

 crystals, from the imbedding substance, appears to be of a petro- 



* Vide T. G. Clemson's paper on Analysis of divers Mineral Substances, Jour- 

 nal of the Fr anklin Institute. 



