﻿lxvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I905, 



mass from top to bottom without any lines of division. These breaks or 

 divisions may as distinctly mark a cold period as though they had been 

 occupied by beds of sandstone. The marine creatures ceased to exist, and when 

 the rough surface left by their remains became smoothed down by the action of 

 the waves into a flat plain, another bed would begin to form upon this floor so 

 soon as life again appeared.' 



We need not necessarily assume with Croll that the beds of 

 limestone represent warm interglacial periods, and the planes of 

 division cold periods of glaciation, for change of climate, apart from 

 glaciation, might be sufficient to cause the temporary disappearance 

 of the rock-builders. I shall presently recur to this question of 

 climatic change ; I have mentioned it here, because it appears to me 

 to be worth while to make a series of observations of widespread 

 masses of pure limestone, with the object of noting the vertical 

 distances which separate a series of dominant bedding-planes in 

 such masses of pure limestone, like those to which Croll refers, and 

 also the horizontal distances to which these dominant planes can 

 be traced. Comparative regularity in the thicknesses of successive 

 strata which can be traced far laterally, implies the comparatively- 

 regular occurrence of the cause. It is these regularly-recurrent 

 events which, if they happened, we should wish to detect. 



(2) Differences in the Characters of Successive Deposits. 



We noted that the lines of the geograms would be blurred, 

 owing to vertical and lateral passage from one kind of sediment 

 into another. A sandstone, for instance, will not necessarily be 

 markedly separated from a shale, but may gradually pass into 

 it (either vertically or laterally) by the mixing of argillaceous 

 with the arenaceous material in ever-increasing proportion. 



But, in addition to this, there is another reason why the lines of 

 the geograms should be less defined than those of a meteorogram. 

 The lithological characters of a group of sediments are the product 

 of many factors; and the conditions which produce similarities of 

 one kind in these sediments may have a lateral distribution different 

 from those which cause similarities of another kind, and the two sets 

 of conditions may also change in any area at different times. Thus, 

 a sandstone may be accumulating in one area, while a mud is being 

 deposited in an adjoining tract. A supply of mica may be carried 

 into these two tracts simultaneously, giving rise to a micaceous 

 sand in the one, and to a micaceous mud in the other. The supply 

 of mica might conceivably be cut off, though the materials which 



