
492 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. — 
takes place at the present time on lake-bottoms, estuaries, or sea- 
margins to be assured that. this is now, and must always have been, © 
the law of sedimentation. , 
But while all sedimentary deposits must be regarded as essen- 











Fic. 209.—Svuccrssion or Burren Coan Growras AND Erect TREE-STUMPs, SYDNEY 
Coau-Fretp, Care Breron (R. Browy)." 
a, sandstones; b, shales; ¢, coal-seams; d, beds containing roots and stumps 77 sélu. 
tially local, some kinds possess a far greater persistence than others. 
As a general rule it may be said that the coarser the grain the more 
local the extent of a rock. Conglomerates are thus by much the 
most variable and inconstant of all sedimentary formations. ‘They 
suddenly sink down from a thickness of several hundred feet to a 
few yards, or die out altogether, to reappear perhaps further on, in 
the same wedge-like fashion. Sandstones are less liable to such 
extremes of inconstancy, but they too are apt to thin away and to 
swell out again. Shales are much more persistent, the same zone 
being often traceable for many miles. Limestones sometimes occur 
in thick local masses, as among the Silurian formations, but they 
often also display remarkable continuity. Three thin limestone 
bands, each of them only two or three feet in thickness, and separated 
1 See R. Brown, Quart. Journ. Geol. Soc. vis p. 115, and De la Beche, * Geol. 
Observer,” p. 505. 
