CHEMICAL WORK. 131 



be, its spacious chambers, high Avater-falls, and free-flowing rivers. The 

 flowing waters sometimes work also by abrasion ; but there is usually little 

 loose material to transport for the purpose of abrasion. 



In a similar way limestone cliffs have been chiseled into ranges of 

 turrets, and deep recesses and channels made for rivers through lime- 

 stone strata. 



The excavation of the lagoon basins of coral islands has been attributed 

 erroneously to erosion by the carbonic acid of the sea water. 



(d) Except for surface erosion, limestone consisting of pure calcite, free 

 from iron sulphides, is a durable rock, whether uncrystalline or crystalline, 

 as in the case of the Carrara marble, of which such marvelous structures as 

 the Milan cathedral have been made. But a magnesian limestone or dolo- 

 myte, when crystalline, is often easily destructible, because, as already stated, 

 the porous rock is likely to contain disseminated calcite ; and as this is 

 more soluble than dolomyte, percolating waters carry it off, leaving the rest 

 in the state of sand — a bad condition for the marble temple that may be 

 made of it. The presence of the calcite can be detected only by observing 

 whether, at any exposure of a layer in the region of a quarry, it is 

 turning to sand. 



Polished limestone marble containing any chert or other hard mineral, 

 if employed in out-door ornamentation or on monuments, is sure to weather 

 rough and become unsightly, and the chert may be made to stand out in 

 ragged points or knobs. Even the vertical movement of the atmosphere 

 over polished marbles will in time take off or dim the polish. 



(e) Since carbonic acid attacks feldspar as well as other minerals, this 

 agency, and that of oxidation, leave scarcely any kind of rock safe against 

 destruction. Those are safest that are free from iron sulphides, and 

 especially those that are so fine-grained and compact that water cannot 

 gain access. Hence, the method of testing rocks for porosity by ascertaining 

 how much water they will absorb in 24 hours is excellent. Some slate 

 rocks are very durable because of their fine grain and the absence of any 

 soluble minerals. Some granites absorb little water, some very much ; and 

 the latter are easily destructible. 



3. Constructive effects. — (a) Calcareous deposits. — The most familiar 

 deposits of this kind are the stalactites and stalagmites of caverns, dripstone 

 formations ; so-called because made by the calcareous waters dropping from 

 the roofs. The " Gibraltar rock " is stalagmite. Still more interesting are 

 the travertine or tufa deposits of streams. Leaves, nuts, and stems are often 

 petrified by calcareous waters. 



The travertine of Tivoli, near Rome, constitutes a large deposit along the 

 Anio, whose waters are there strongly calcareous. Along Gardiners Eiver, 

 in the region of the Yellowstone Park, thick limestone deposits have been 

 made, as is well illustrated and described in the Reports of Hayden's 

 Geological Survey of the Territories. The calcareous waters, in descend- 

 ing the slopes of the hills, have made a series of parapets at different 



