Trans. N. V. Ac. Set. 38 Nov. 20, 



the softer material, leaving the harder ledges above. After a while the 

 weight of the over-hanging mass causes it to fall. Thus valleys are 

 made with precipitous sides and encumbered by rocky debris. Thus 

 also natural bridges are left, like that famous one in Rockbridge County, 

 Virginia, which is but a remnant of the roof that onceover-arched the 

 valley. 



The homogeneity of the limestones of the Ohio Valley is extremely 

 favorable for the formation of extensive caverns. It is estimated that 

 there are 8000 square miles of cavernous limestone in Kentucky alone. 

 I remember seeing, in a marble quarry in that region, a block without 

 a flaw, 150 feet long by 30 feet square, cut simply to show what could 

 be done. 



The thickness of the sub-carboniferous limestone, from which Mam- 

 moth and Wyandot Caves are excavated, varies from 100 to i.oco feet. 

 This makes possible the formation of long, winding avenues, with here 

 and there extraordinary enlargements, spacious halls, deep pits and lofty 

 domes, cutting through all galleries from the surface down to the drain- 

 age level. 



The Virginia caves, on the other hand, of which Luray Cave is the 

 noblest specimen, are examples of excavations from rock tilted and 

 fractured by upheavals, the resulting seams having often been after- 

 wards filled with crystalline material. Hence, instead of well defined 

 arcades, and symmetrical halls and domes, there are extremely irregular 

 rooms, adorned with such a profusion of stalactitic ornamentation, that 

 in Luray Cavern, there seems to be hardly a square yard left bare 

 showing the naked rock ! 



Subterranean scenery is also effected by various mineralogical modi- 

 fications. The sharp, glassy needles of obsidian, in the lava cave of 

 Surtsheller, differ greatly from the snowy embellishments found in the 

 Grotto of Antiparos, carved from statuary marble. And again, the 

 majestic columns of basalt, that guard Fingal's Cave, strike the eye 

 differently from the frowning arches of the granite caves of Norway or 

 of Maine. 



Certain metallic ores may stain the cavern walls red, yellow, blue or 

 black. And when this incrustation is broken through by the efflor- 

 escence of the shining white crystals of the sulphate of lime, of mag- 

 nesia, or of soda, remarkable effects are produced. 



Gigantic silhouettes seem to be cut from the ceiling of creamy 

 limestone ; grotesque forms start out from the darkness; grim spectres 

 seem to wave their shadowy arms. These illusions are purely miner- 

 alogical. A strangely beautiful example of such transformations may 

 be seen in the so-called Star Chamber of Mammoth Cave ; where the 

 gray walls support a lofty ceiling coated with the black oxide of man- 



