
110 STRUCTURAL AND FIELD GEOLOGY 
of stratification amongst such accumulations will represent longer 
intervals than in the case of estuarine and littoral deposits. Should a 
pure marine limestone of some thickness, for example, be immediately 
underlaid and overlaid by thick argillaceous shales, as in the accompanying 
illustration (Fig. 5), we should be justified in assuming that the planes 
of stratification indicated lengthy intervals of time. Such an alternation 
of deposits would necessarily imply certain geographical changes, and 
these, as a rule, are only developed very slowly. We should infer that 
some change of conditions had arrested the deposition of muddy 
sediment represented by the lower beds of shale (s#!)—either the source 
of supply was cut off, or the 
current which brought the sedi- 
ment had lost its force, or was 
diverted in some other direc- 
tion. The presence of thick pure 
limestone (), consisting of the 
débris of corals and _ other 
marine organisms, points to a 
FIG. 5. SHALES AND LIMESTONE. ' long-continued period during 
shi, sh2, shales; 1, limestone. which the water remained clear. 
Then the sudden appearance of 
the overlying shales indicates a resumption of the conditions which 
obtained during the deposition of the lower shales. Possibly the alter- 
nation of beds may point to crustal movements. It may be that the floor 
of the sea subsided so as to carry it beyond the reach of mud-transporting 
currents, and after a prolonged period of rest, during which the corals and 
their congeners flourished, a new crustal movement in the opposite direc- 
tion brought the same region again within the influence of currents laden 
with fine sediment. Explain the alternation of strata as we may, it is 
obvious that the planes of stratification in this case indicate more or less 
prolonged intervals. Geologists do not doubt that in some cases these 
planes may well represent a longer period of time than was required for 
the accumulation of the various strata which they separate. 
Extent and Termination of Beds.—Fine-grained deposits 
usually have a wider extension than coarse-grained accumula- 
tions. This is quite in keeping with what we know of the 
distribution of sediments in the lakes and seas of our own 
day. When a river enters a lake or estuary, the force of 
the current is immediately checked, and the heavier and 
coarser materials, gravel, etc., are at once thrown down. Grit 
and sand are swept out to a greater distance, and more 
extensively distributed, while the finest particles travel further 
still, and are spread over a yet wider area. Practically the 
same kind of sifting-out of sediments is effected by waves 
and tidal currents along an open coast-line. Banks of shingle 


