INTRODUCTION. 



39 



bury and hide from sight the more consolidated and regular masses above described. The 

 rocks are rounded, their sharp edges having been evidently worn off by friction, and the earthy 

 materials have as evidently been produced by the grinding down of the rocky fragments during 

 the same process. Everything here bears the mark of the action of powerlul floods of water, 

 which have swept away and ground up or rounded the fragments of native rock ; as this forma 

 tion is the elTect of some great deluge, it is called the diluvial formation. Sometimes the 

 masses which have been moved in this manner from their native beds are of great size, and it 

 is possible to trace the direction of the diluvial current by ascertaining their original situation. 

 By this process it is shown that such a flood has swept over the United States from northwest 

 to southeast, scattering over the surface immense numbers of those lost 7-ocks or travelers, as 

 they are often called by the people, or bowlders or erratic blocks, as the geologists term them, 

 to distinguish them from rocky masses in place, that is, in their native beds. 



8. Caverns. Fissures and cavities are sometimes found of great extent, forming vast caverns 



in the bowels of the earth, which 



are either empty or filled with water. 

 Instances of these on a great scale 

 occur in the limestone strata of 

 many countries, especially of Car- 

 niola and the United States. The 

 cavern of Adelsberg, in Carniola, 

 extends several leagues in its va- 

 rious branches, and contains a sub- 

 terranean lake and river ; the not 

 less remarkable Mammoth cave of 

 Kentucky has been traced several 

 miles, and is also in limestone. 

 Some of these caves are brilliantly 

 ornamented with crystals of every 

 shape and hue ; such is Weyer's 

 cave, in Virginia, which consists of 

 numerous large apartments and gal- 

 leries filled with stalactites, or crys- 

 taline masses resembling various 

 natural and artificial objects. The 

 celebrated spar cave, of the Isle 

 of Skye, is in sandstone ; there is 

 also one in a slaty rock in ihe island 

 of Thermia:, and the great Icelandic 

 cave of Surtshellir is in lava. This 

 last is nearly a mile in length, and 

 appears to have been formed in 

 flowing lava by the disengagement 

 of gaseous matters, as it has all 

 the appearance of being the con- 

 sequence of an enormous air-bub- 

 ble, having its roof ornamented 

 with projections of black slag. The 

 celebrated Columnar cave on the 

 isle of Stafia, well known as Fin- 

 gal's cave, is in trap-rock, 

 acidiferous, earthy, inflammable, and 



Section of a Cavern in Europe. 



Fingal's Cave, Slajfa. 



Minerals are divided into four classes 



9. Minerals. 

 metallic. 



Of the first class, we may notice limestone, calcareous spar, or carbonate of lime, that is, 

 lime combined with carbonic acid. It is the most abundant of any known mineral substance 

 on the earth. It is one of the principal constituents of rocks. It is also found in its pure 

 state in immense beds or banks ; of stratified earth it forms more than half. It is also found 

 combined with clay, and thus constitutes the various marls. It forms vast beds, in the shape 

 of chalk, mingled with large masses of shells. 



