WATSON— NOTES ON METALLIFEEOUS SADDLES. 



367 



clianges may be attributed in general terms to subterranean disturb- 

 ances. A glance at tbe geology of tbe neighbourliood to which our 

 observations apply, is sufficient to show by surface evidence alone 

 that the country, to anglicise a useful French geological term, is 

 accidented ; 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 formed over some old focus of disturbance ; 

 and in this way the siHceous beds may be considered as remotely 

 related, as once before observed, to the chain of first causes in the 

 history of events. We may infer, then, that the 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 

 elevatory forces uplifting wide tracts of country with energy varying 

 in intensity and unequally ap^^lied. 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, depression. And if we reflect for a 

 moment on the cha^nge of volume that takes place in solids when 

 affected by high ranges of temperature, not sufficient to produce in 

 degree any other change, it vnll be evident that any large mass of 

 strata caiTied below the stratum of invariable temperature and ex- 

 posed, it may be, for centuries to a regular, though not necessarily 

 very high, temperature, vrill, if afterwards elevated, display marked 

 effects, partly mechanical, partly chemical, of the action to which it 

 has been subjected. The expansion of rocks in the direction of theu- 

 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 temperatm^e 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 comijressible strata of the 

 Hmestone, 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 regular temperatm^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 tempei^atm-e lies in latitudes forty-eiglit degrees and 

 fifty-tTTO degrees north, at a depth of about sixty to sixty-four feet. 



