Parti] —- STRATIFICATION. 475 
break more readily across them than in their direction. More usually, 
however, the planes of lamination serve as convenient divisional 
surfaces by means of which the rock can be split open. ‘The cause 
of this structure has been generally assigned to intermittent deposit, 
each lamina being assumed to have partially consolidated before its 























































































Fic. 182.—Sra-Ciirr suowine A Serius or Srratirtep Rocks (B.). 
successor was laid down upon it. Mr. Sorby, however, has recently 
suggested that in fine argillaceous rocks it may be a kind of 
cleavage-structure (see p. 310) due to the pressure of the overlying 
rocks with the consequent squeezing out of insterstitial water and 
the rearrangement of the argillaceous particles in lines perpendicular 
to the pressure.’ 
Much may be learnt as to former geographical and geological . 
changes by attending to the characters of the strata. In Fig. 183, for 
example, there is evidence of a gradual diminution of movement in 
the waters in which the layers of sediment were deposited. The con- 
glomerate (a) points to currents of some force; the sandstones (be d) 
mark a progressive quiescence and the advent of finer sediment; 
the shales (e) show that by the time they were formed, only very 
fine mud was borne along; while the shell-limestone (/) proves that 
the water no longer carried sediment, but was clear enough to permit 
of an abundant growth of marine organisms. ‘The existence, 
therefore, of alternations of fine laminz of deposit may be conceived 
as pointing to tranquil conditions of slow intermittent sedimentation, 
where silt has been borne at intervals and has fallen over the same 
area of undisturbed water. Regularity of thickness and persistence 
of lithological characters among the lamin may be taken to indicate 
periodic currents, of approximately equal force, from the same 
quarter. In some cases successive tides in a sheltered estuary may 
have been the agents of deposition. In others the sediment was 
doubtless brought by recurring river-floods. A great thickness of 
laminated rock, like the massive shales of Paleozoic formations, 
suggest a prolonged period of quiescence, and probably in most 
cases, slow, tranquil subsidence of the sea-floor. On the other hand, 
1 Quart. Journ. Geol. Soc. xxxvi. p. 67 (1880). 
