86 CHEMICAL DEPOSITION. 



cleavage a relation arising from the displacement of the strata by 

 axes of elevation and depression. Parallel to these axes is the "strike " 

 or horizontal line on the surface of the strata ; if this be taken on a 

 great scale and the " strike " of the cleavage (similarly denned) be 

 compared with it, the direction of each is found to be the same, or 

 nearly so ; in other words, the cleavage edges on the surface of the 

 strata are horizontal lines (s-s in fig. 36). The direction, then, of 

 the cleavage in a given district is dependent in a general sense on 

 that of the axes of earth-flexure in that district ; but the inclination of 



the cleavage has no necessary 

 known relation to that of the 

 strata (fig. 38) ; beyond this, 

 that the dip of the strata 



Fig. sS.-Paralle^cleavagem^contorted slates of being mo d era te, that of the 



cleavage is usually greater. 



In a country where the strata are much undulated, the cleavage may 

 be and mostly is in parallel planes. 



Local Changes of Internal Structure. We must defer to a later 

 page the theoretical considerations which arise out of these facts, 1 and 

 some other valuable data, collected by Mr. Sharpe, and later still by 

 Mr. Sorby; but though a little out of place, we cannot forbear to 

 add here a short notice of facts known in Switzerland, which dis- 

 tinctly prove one of the effects of heat upon common argillaceous 

 shales, to be the alteration of their structure, so as to give a real ver- 

 tical cleavage to a mass of horizontal laminae of clay, as well as that 

 induration which belongs to slate. The Lias shales of the Alps are so 

 altered by proximity to the igneous rocks of that region, that in several 

 places in and near the Valley of Chamouni they are commonly mis- 

 taken by modern tourists for genuine slates of the Primary system, 

 and were always described as such by the older writers. This demon- 

 strates that cleavages, and other peculiarities of structure, not pro- 

 duced in rocks by water, nor coeval with their deposition, have been 

 occasioned subsequently, chiefly by the agency of pressure or molecular 

 rearrangement. 



Composition of Strata. 



Chemical Deposits. Under different circumstances water, at 

 certain temperatures, and by the help of soluble acids or alkalies, 

 dissolves various mineral substances. When, by evaporation, loss 

 of heat, or a change in the composition of the liquid, these sub- 

 stances are no longer capable of remaining in solution in it, they 

 separate in a more or less crystallised form, and the deposit which 

 they occasion is termed a precipitate. By such processes lime, mag- 

 nesia, and other earths and metallic oxides are first dissolved in 

 water, and afterwards separated from it. In this way calcareous marls 

 and irregular accumulations of limestone, in lakes and in the course 



1 These remarks on cleavage are based on observations by Prof. Phillips, 

 and were mostly published in Encyclo. Metrop., 1833 > Guide to (Geology, 1834-6- 

 51 ; Treatise on Geology, 1853 ; Brit. Assoc. Report, 1843. 



