420 DESCRIPTIONS OF BOCKS. 



anhydrous trap now undergoes shows that such waters do not make 

 their way down : and moreover the results could not have been pro- 

 duced without heat. The trap has not been subjected to a metamor- 

 phic process ; for the Triassic beds are unaltered sandstone. The water 

 was not from the deep-seated source of the erupted trap, for, if so, the 

 dikes would have been all of one kind, instead of being part hydrous 

 and part anhydrous, and the former locally distributed just as subter- 

 lanean streams of water are likely to be. 



In the case of hydrous met amorphic rocks, whether con- 

 taining chlorite, talc, or a hydrous mica, the hydrous min- 

 erals were, with rare exceptions, made at the time of the 

 crystallization, and are not a consequence of subsequent al- 

 teration. 



3. Durability ix Socks. — Durability in a rock is due 

 largely (1) to compactness and fineness of texture; and 

 (2) to the absence of any ingredient or mineral that is liable 

 to oxidation. As far within a rock as water and air can gain 

 access, degradation will always be going on, and most rapidly 

 in all crevices along their walls. Alternate melting and freez- 

 ing will be one means of destruction ; direct chemical action of 

 moist air, especially the carbonic acid it contains (p. 108), 

 another ; the wedging apart of grains caused by the slightest 

 deposits and oxidations, through infiltrated waters, another. 

 In granite the carbonic acid may take the alkalies out of 

 the feldspar, and so occasion the destruction of the rock. 



Hence the practice of testing the durability of a stone for 

 architectural purposes, by putting it into water, and then 

 weighing it, after some days of exposure, to see whether it 

 lias gained in weight, is a good one. 



Fineness of grain gives further protection against destruc- 

 tion. Alternate heating and cooling in the daily passage of 

 the sun is a destroying agency of great effect, especially on 

 coarse-grained kinds. Eocks have often retained the glacier 

 markings upon them perfectly fresh until now, when they 

 have had a covering of two or* three feet of earth; and they 

 have lost such markings after a few years of exposure. This 

 happens often where there is no true decomposition or ox- 

 idation of the surface portion of the rock, and must be due 

 largely to the expansion and contraction caused by changing 

 temperature. The finer the grain of the rock the less the 

 chance for this action. There is no more durable rock than 

 a roofing-slate of good quality. Granites, when well pol- 

 ished, will usually resist long all weathering agencies. 



