lvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 
deposits, we see so many causes for change and modification arising 
from physical conditions, that we could scarcely expect uniformity 
of result, even upon extended coasts ranging along the same pa- 
rallels of latitude, or what might amount to the same thing, as 
regards temperature, if an internal heat of the globe may have 
been so felt at these geological times as to have given a certain tem- 
perature to the ocean, higher generally than at present. Always, 
under the latter hypothesis, duly considermg the modifications m the 
position of such waters from the differences in specific gravity arising 
from differences of temperature. We have to look also to bays, 
headlands, shoals, deep waters, and all the modifications now obser- 
vable, including tidal action in some places greater than at others,— 
estuaries and bodies of fresh water flowing over the heavier waters of 
the sea, until their power to do so ceased, and any detrital matter 
they may have held in mechanical suspension was deposited far out- 
side the localities where the sea-bottoms may have been coated by 
mineral substances brought directly over them by river action. 
We have also to look to a fair proximity to land for the deposits 
generally, if we are to have any regard to the kind of marine mineral | 
accumulations now observable, or which are at all probable. When 
we consider the thousands of feet of thickness which have to be given 
to these older deposits, we have to weigh the modifying imfluences 
exerted upon the life, existing upon the sea-bottoms of the time, in 
consequence of the filling-up of the deeper waters, unless we infer 
that the sea-bottoms were depressed as the mineral matter and the re- 
mains of animals were, bed upon bed, deposited upon them. Neither 
- should we neglect, when great distances are under consideration, the 
chances of unequal depressions or elevations of the sea-bottoms and 
dry land durmg the lapse of the geological time when any large 
amount of accumulations in the sea may have been effected. 
Independently of these physical considerations, we must not forget 
the time required for the dispersion of the species themselves, if 
specific centres are in any way to be regarded, even as a convenient 
hypothesis, and the chances that the germs of these species, floating 
away from the parent stock, only here and there, and over very un- 
even areas, found the physical conditions suited to them. Then also 
we have to take ito account the views of the naturalist examining 
specimens obtained from the rocks which, or the remaiming visible 
parts of which, represent to us the mud, silts, sand and gravel of 
these ancient times. He may be, as many paleontologists have been, 
inclined to attach a higher value to certam differences and resem- 
blances of general forms, or of parts of forms, than another, and 
hence species may be so greatly multiplied in the cabinet, that our 
books may represent them to have been limited to a geological time, 
corresponding only with a given mineral accumulation, while the 
study of a greater number of specimens among the rocks themselves 
may not warrant these conclusions. 
We have also to regard another point of no small importance, 
which is that our classifications of rocks in one part of the earth’s 
surface, even taking an area equal to that occupied by Europe, is 
