58 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



ORIGIN OF CLAY SLATE AND PHYLLITE. 



The origin of the clay slates of the Mansfield formation is probably to 

 be looked for in the disintegration and decay of the Archean granite, and 

 the subsequent metamorphism of the resulting clay. For between the 

 granites and the slates no other rock masses are known to have existed from 

 which the clay could have been derived. The phyllites are presumed to 

 have resulted from the metamorphism of the clay slates. 



PRESENT COMPOSITION NECESSARILY DIFFERENT FROM THAT OF ROOK FROM 



WHICH DERIVED. 



It is a well-recognized principle of rock weathering that in the altera- 

 tion of rocks near the surface of the earth there is a relatively rapid 

 diminution in the quantity of the more soluble constituents. Hence a clay 

 shows a lower percentage of alkalies and alkaline earths than is found in 

 the parent rock, with an increase in the percentage especially of alumina 

 and water. This relation is made clear by Adams in a statement of the 

 comparison of the composition of certain slates and gi-anites:^ "On com- 

 paring the analyses of a series of granites with those of a series of slates, 

 as, for instance, those given in Roth's 'Gesteins Analyzen,' the latter are seen 

 to be on an average considerably higher in alumina and much lower in 

 alkalies, while at the same time they are lower in silica, wliich has been 

 separated both as sand and in combination with the alkalies which have 

 gone into solution, and in most cases contain more magnesia than lime 

 instead of more lime than magnesia, as is usual in granites." Adams con- 

 cludes further, after a comparison of the alkalies in the slates and granites, 

 that "The slates thus contain on an average about two-thirds of the amount 

 of alkali present in the average granite."^ An examination of series of 

 analyses of granites shows that while the percentages of soda and potassa 

 vary considerably, now the one being predominant, now the other, on the 

 whole in the typical granites the potassa is higher than the soda.^ This is 

 the relation which we would expect in the case of an ideally pure granite, 



' A further contribution to our knowledge of the Laurentian, by F. D. Adams : Am. Jour. Sci., 3d 

 ser., Vol.L, 1895,p. 65. 



■^Loc. cit.,p. 65. 



' Zirkel states that iu the weathering of granites the soda is much more readily removed than 

 is the potassa: Lehrbuch der Petrographie, Vol. II, 1894, p. 32. 



