WATSON-- NOTKR ON METALLIFEROUS SADDLKS. 
8G7 
changes may be attributed in general terms to subterranean clistui'b- 
anees. A glance at the geology of the neighbourhood to which our 
obsei'\'ations apply, is sufficient to show by surface evidence alone 
that the covmtry, to anglicise a useful French geological term, is 
accklented ; and we may consider the whole of the changes, whether 
mechanical or chemical, to have occurred subsequently to the first 
formation of the rocks. What is more, if we attribute the presence 
of the bands of chert to the separation of silica from hot springs 
during the deposition of the strata, we may fairly assume that the 
rocks themselves are foi-med over some old focus of disturbance ; 
and in this way the siliceous beds may be considered as remotely 
related, as once before observed, to the chain of fir&t causes in the 
history of events. We may infer, then, that tlie dislocations of strata 
giving rise to the irregularities of the surface, and probably the 
greater part of the veins, fissures, and joints have resulted from 
elevatoiy forces uplifting wide tracts of country with energy varying 
in intensity and uneqiially applied. For the plication of the strata, 
the in-filling of the ores, and the general molecular changes in the 
rock composing the saddles, we must look to another cause, the 
opposite of the above, namely, depi'ession. And if we reflect for a 
moment on the change of volume that takes place in solids when 
affected by high ranges of temperature, not sufficient to produce in 
degree any other change, it will be evident that any large mass of 
strata carried below the stratum of invariable temperatvire and ex- 
posed, it may be, for centuries to a regular, though not necessarily 
very high, temperature, will, if afterwards elevated, display mai'ked 
effects, partly mechanical, partly chemical, of the action to wdiich it 
has been subjected. The expansion of rocks in the direction of their 
length has been made out for each increment of one degree of Fah- 
renheit above the ordinary mean temperature at the surface, and by 
these experiments we learn that if a mass of limestone one hundred 
miles in length be removed by subsidence to a depth of about two 
miles below the stratum of mean temperature*, where it will 
encounter an elevation of one hundred and eighty degrees Fahren- 
heit beyond the mean temperature of its original position, it will 
undergo a lineal expansion amounting to about five hundred and 
forty feet. But such an increase in length could not fail to exert, if 
opposed by any adjoining resistable rocks, aided by superincumbent 
pressure, a very marked effect upon all the compressihle strata of the 
limestone, squeezing and contorting the beds after the manner 
observed ; and at the same time it will be evident that long exposure 
of the rocks to a perfectly even and regnlar temperatui-e would tend 
to produce great molecular change, and probably a semi-crystalline 
condition. In connection with this therefore, it is well worthy to be 
remarked that these plicated beds which I have been describing are 
perfectly devoid of organic remains, while the beds above and below 
* The stratum of mean temperature lies in latitudes forty-eight degrees and 
fifty-two degrees north, at a depth of about sixty to sixty-four feet. 
