486 Scientific Proceedings, Royal Dublin Society. 
strata, in order to produce correspondence with what is seen in 
other places. 
In the progress of geology, as facts accumulated, it was found 
that the strata forming the crust of the earth were capable of being 
divided into groups distinguishable from one another by special 
characters; and such groups are now known as “ formations.” 
Formations are distinguished from each other both stratigraphi- 
cally and palontologically. Stratigraphically, a typical forma- 
tion, as a general rule, has its beginning and ending distinctly 
marked ; the beds belonging to it lying unconformably on those 
of the older formations, while those of the succeeding ones lie 
_ unconformably on its strata. However, there are cases in which 
the rocks of one formation have “ passage- beds” into those above 
or below them, but such cases are exceptional. 
A formation consists of a greater or less number of sub-groups 
of strata which have certain general characters common to all. 
These sub-groups are often distinct from each other, but usually 
each sub-group graduates into those above and below. Nearly 
always one or more of the sub-groups, which in the entire make 
up a formation, are in some places better developed than the 
others; or, as is often the case, peculiar groups may appear 
exceptionally in certain limited areas and not elsewhere; and 
some sub-groups, which are always traceable in certain regions 
or countries, may be exceptionally absent in others. These 
changes have often been misinterpreted, and some of the geo- 
logists of the present day go on the supposition that a formation 
is incomplete in some particular district unless all the sub-groups 
which can be seen anywhere else are present; also that the 
absence of any one in a certain area is to be necessarily regarded 
as a hiatus in the series, and evidence of a period of time during 
which no strata were deposited (this last conclusion is, of course, 
occasionally correct). In estimating the whole thickness of the 
formation, they therefore include not only the rocks which are 
present, but also those which they suppose to be wanting ; thus 
often making the whole thickness much greater than it may 
really be. 
Such a mode of calculation is, I believe, as a general rule, 
erroneous. The ancient strata were accumulated in more or less 
distinct basins, or what comes practically to the same effect, as 
