a 
498 GHOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV, 
slow deposit and quiescence, almost always of longer duration than — 
would be indicated by an equal depth of sandy strata, pointing to 
more active sedimentation. Thick limestones made up of organic 
remains which lived and died upon the spot, and whose remains are 
crowded together generation above generation, must have demanded 
prolonged periods for their formation. . 
But in all speculations of this kind we must bear in mind that 
the relative length of time represented by a given depth of strata is 
not to be estimated merely from thickness or lithological characters. 
It has already been pointed out that the interval between the deposit 
of two successive laminz of shale may have been as long as, or even 
longer than, that required for the formation of one of the lamine. 
In like manner, the interval needed for the transition from one 
stratum or kind of strata to another may often have been more than 
equal to the time required for the formation of the strata on either 
side. But the relative chronological importance of the bars or lines 
in the geological record can seldom be satisfactorily discussed merely 
on lithological grounds. This must mainly be decided on the evidence 
of organic remains, as will be shown in Book Y. By this kind of evi- 
dence it can be made nearly certain that the intervals represented by 
strata were in many cases much shorter than those not so represented, 
—in other words, that the time during which no deposit of sediment 
went on was longer than that wherein deposit did take place. 
Ternary Succession of Strata.—In following the order of 
sedimentation among the stratified rocks of the earth’s crust, the 
observer will be led to remark a more or less distinet threefold 
arrangement or succession in which the sandy, muddy and calcareous 
sediments have followed each other. Professor Phillips and Mr. Hull 
have called attention to this structure, illustrating it by reference 
to the geological formations of Great Britain, while Professor 
Newberry, Dr. Sterry Hunt, and Principal Dawson have discussed 
it in relation to the stratigraphical series of North America. Ac- 
cording to Mr. Hull a natural cycle of sedimentation consists of three 
phases: Ist, a lower stage of sandstones, shales, and other sedimentary 
deposits, representing prevalence of land with downward movement ; 
2nd, a middle stage, chiefly of limestone, representing prevalence of 
sea with general quiescence and elaboration of calcareous organic 
formations ; 3rd, an upper stage, once more of mechanical sediments 
indicative of proximity to land. Where the strata are interrupted 
by disturbance and unconformability, we may suppose the cycle of 
sedimentation to have been completed by upheaval after prolonged 
subsidence. But where the continuity of the formations is unbroken, 
as it is over such vast tracts in North America, upheaval is not re- 
quired, and the facts seem explicable, as Phillips long ago showed, on 
' Phillips, Mem. Geol. Surv. ii.; “Geol. Yorkshire,” ii.; “Geol. Oxford,” p. 293; 
Tull, Quart. Journ. Sci. July, 1869; Newberry, Proc. Amer. Assoc. 1873, p. 185; 
Hunt, Geology of Canada, 1863, p. 627; Amer. Journ, Sct. (2nd series), xxxy. p. 167; 
Dawson, Q. J. Geol. Soc. xxii. p. 102 ; Acadian Geology, p. 135. 
