238 
series of things.* If the facts forbid the supposition that 
either by the internal heat, or by the accession of stellar heat 
the temperature could be kept the same for an £ “cks b f 
soace of time, then it follows that metamorphism of rocks by 
lfeat cannot have gone on for an indefinite length of time. It 
mult h^had a beginning, it must be tending towards an end. 
Tf all things continue as at present, the denudation of c0 ' 
tinents no^balanced by the decreasing terrestrial heat-effects, 
the resuH wilt be that all the land will be swept into the sea 
But the earth and the solar system ma y “Xat^f le ma'y for 
of which we know little or nothing ; all that we see may, i 
aucTt we know, be modified at any moment by an unsuspected 
expression of highest law, saying, “ Thus far shalt thou go and 
no farther ! ” 
SEDIMENTARY ROOKS. 
27. The lowest rocks with which we are acqm Xedthf 
igneous substances, are the coarse hard 
Laurentian. They are, like gram e, c ara |, nrn uf cn de-rock 
silica They consist of gneiss, mica-schist, hornbie > 
quartz-rock J felspar, and limestone. They are in Canada 30 000 
feet thick and are found m various parts of the world, in y 
are 1 the most ancient of the rocks with laminated structuie, an 1 
were S lately, termed primitive. As a 
unlike any other. The quartz rocks of the lo ^ La ~“ 
28 The next formation in the ascending scale is 
,Z, X which, for o« P .~»t |»« >» 
overlying Silurian and Bewman, forming together a va ^t serie 
at least 70,000 feet thick, differing greatly m its composition 
from the Laurentian by, amongst °ther pecu harries, the 
nresence of a larger quantity of alumina and less of silica, lh 
difference has furnished the materials for the development of 
slaty cleavage. Cleavage is not wholly ab sent f ' the e 
below nor from certain rocks immediately above, but it nas u 
chief ’home amidst these Cambrian, Silurian, and Devoma 
* “With respect to inorganic matter, the theories of nnifomity 
a* *** 
vol. viii. p. 56 ; and Cambridge Essays, p. 215. 
