GENERAL VIEWS. 
175 
at the trouble of the necessary discussions, and thus vast collections of 
facts become almost useless, and years of labour end with no im- 
portant result. 
Succession of deposits. — Fixing our attention on the most striking 
feature of the mountain limestone deposits of the North of England, 
the repeated succession of nearly similar combinations of limestone, 
gritstone, and shale, which constitute them, we shall find two con- 
clusions inevitable; first, the mineralogical differences of the parts 
of the series of deposits must be referred to different general agencies, 
or local conditions ; secondly, these different agencies or local influences 
predominated periodically. Neither of these propositions needs further 
proof than the mere inspection of the diagrams on p. p. 37 and 38 ; 
the successive different deposits there indicated are the effects of dif- 
ferent agencies or conditions, successively predominating, and the re- 
petition of such successions proves the periodical predominance of the 
causes. Turning next to the results already partially disclosed, p. p. 
32, 46, &c. of the unequal distribution of different members and 
groups of strata, we find the predominance of certain strata, particu- 
larly the gritstones and shales and certain limestones, to be a limited 
phenomenon, spreading from certain centies ox lines of piincipal inten- 
sity and vanishing in particular directions in a definable ratio. Hence 
I adopt a third conclusion, that strata and groups of strata so cir- 
cumstanced are the effects of local physical conditions, while others, 
as the lower limestone for instance, being found with very similar 
characters in many parts of England, Ireland, and some other parts 
of Europe, appear to indicate more general and continual agencies. Yet 
even with respect to these more general deposits, it is sufficiently 
proved (p. p. 84 and 86,) that they are also locally variable and liable 
to lose their character of continuous deposits and to assume that of 
periodicity. Hence the primary object of theory is to discover the local 
centres and lines of principal intensity of the several agencies or physical 
conditions concerned. 
i 
Lines and centres of greatest and least thickness. — It has been al- 
