130 



DYNAMICAL GEOLOGY. 



Spondylus, Patella, Fusus, Purpura, and Littorina, in which the inner pearly 

 layer is aragonite, and the outer calcite. The shells of most Gastropods 

 and of Cephalopods are aragonite ; and Corals, including the Millepores, are 

 mainly so ; while shells of Khizopods, Echinoderms, and Brachiopods consist 

 of calcite. 



Further, if the limestone contains iron or manganese combined with the 

 calcite, carbonated water will bring the iron to the surface, or the iron car- 

 bonate, or the manganese, for oxidation, weakening and discoloring the rock. 

 The action on feldspar, above mentioned, is a common means of destruction 

 in the case of granites and related rocks. 



(b) Process of draini7ig limited. — But it is also to be observed that these 

 effects occur only so far as the rocks are porous. The fossils of compact 

 argillaceous sandstones and shales — common kinds of f ossilif erous rocks 

 and some dating from the Cambrian — are seldom drained out or injured 

 at all by infiltrating waters, except when near the surface. The iron and 

 manganese taken out of some crystalline limestones are removed only for a 

 short distance inward ; but the process destroys the limestone as it eats in^ 

 and is thus enabled to erode farther. Deep below the surface the same rocks 

 are solid and not discolored. All deep-water rocks are moist, but the moist- 

 ure is ordinarily stationary unless a surface drought reaches downward, or 

 an invasion of heat comes upward from below, when the moisture thus lost 

 may be later replaced. Even beds of salt in subterranean rocks are not 

 dissolved away. 



(c) Surface erosion. — Waters containing carbonic acid or humus acids 

 eat away the surface of solid limestone, fluting precipices, widening crevices, 

 excavating caverns. They often leave calcareous fossils projecting slightly 

 above the svirface, and develop with great perfection silicified kinds. The 

 length of the caverns thus made in the Carboniferous limestone of Kentucky,, 



Making of caverns in limestone. Shaler. 



a rock 200 to 1000 feet thick, is estimated by IST. S. Shaler to amount to 

 100,000 miles. The work is begun by the descent of waters along joints 

 in the rock, whenever there is a chance for discharge below, by running 

 down or trickling along between layers of the limestone. The process and 

 result are illustrated in the above figure by Shaler. In the movement of the 

 waters, the fissure or joint (B) becomes enlarged to a "sink-hole," and exca- 

 vation begins between the layers. The end is a great cave, having, it may 



