RELATIVE AGE OF STRATA. 



47 



carve out hollows representing valleys, cutting through inclined strata 

 at various angles with the line of dip and line of bearing : by this 

 means, he may gain a more correct idea of the varied phenomena of 

 stratification, both in mountains and valleys, than the most elaborate 

 descriptions can convey. 



The appearance of contorted stratification, in the calcareous moun- 

 tains of the Alps, is frequently, an optical illusion. Strata, which 

 have originally enfolded a mountain like the coats of an onion, have 

 fallen off in curved lines, leaving waving edges, overlapping each oth- 

 er, as represented Plate II. fig. 5. Suppose indented sections were 

 made in the side of an onion, the edges of the different indented rinds 

 would present similar contortions. 



Inequalities in the general curvature of the beds may have occa- 

 sioned them to break off in this manner. The Montagne de Tnille, 

 near Montmelian, in Savoy, of which a plate is given in the third vol- 

 ume of Saussure's Voyages dans les Alps, offers an instance of this 

 apparent contortion, which Saussure considers as almost inexplica- 

 ble. I examined this mountain from various stations with much at- 

 tention, and am convinced that the conlortions are only illusory, and 

 are not like the real contortions, which the lower beds of transition 

 limestone, in this country, frequently present on a small scale. In 

 certain situations in the Alps, however, the strata have evidently 

 been raised by some violent convulsion, and have been bent by the 

 resistance which they have offered to the moving cause. Of this a 

 remarkable instance may be seen in the Baltenberg mountain, at the 

 head of the lake of Brientz, of which I have given a description and 

 drawing in the second volume of my Travels in the Tarentaise. 



The strata of secondary rocks belonging to the same formation, 

 frequently preserve nearly the same thickness for a considerable ex- 

 tent, and are arranged conformably over each other, except in situa- 

 tions where their regularity has been disturbed by rents or fractures. 

 In these secondary conformable strata, the order in which they suc- 

 ceed each other indicates their relative ages; but this rule cannot be 

 extended to all classes of rocks. 



No inference can at first appear more legitimate than this: — " The 

 rock which supports another must be older than that which rests up- 

 on it, if their original position has not been changed." But this con- 

 clusion, when examined with attention, will fairly admit of doubt^ 

 with respect to those rocks which are crystalline like the primary. 

 These were either formed by chemical affinity from a state of solu- 

 tion, or by crystallization from a state of fusion : — If by the latter 

 mode, all the different beds may have been arranged at the same 

 time, and the upper and lower rocks may have a cotemporaneous 

 origin. If a mass of melted matter, from a furnace, cool slowly, the 

 internal and external parts will vary, both in their physical and chem- 

 ical properties ; but it cannot, on this account, be said that the lower 

 I part is older than the upper. Butj strata deposited by water were, 



