420 DESCRIPTIONS OF ROCKS. 
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 ofthe erupted trap, for, if so, the 
dikes would have been all of one kind, instead of being part nydrous 
and part anhydrous, and the former locally distributed just as subter- 
ranean streams of water are likely to be. 
In the case of hydrous metamorphic 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 IN Rocks.—Dnurability 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 withinarock as water and air can gain 
access, degradation will always be going on, whatever the 
rock. ‘The alternate melting and freezing of the cold sea- 
son will be one means of destruction; and direct chemical 
action of the moist air, and especially of the carbonic acid it 
contains (p. 108), will be another. This carbonic acid may 
take the alkalies out of the feldspar of a granite ; and with 
the commencement of the action the rock will begin at sur- 
face to fall to pieces, or at least to show weakness. 
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 
has 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. Rocks 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 
iargely 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. 
