494 Scientific Proceedings, Royal Dublin Society. 
sequences seems to be proved; for whether the Rhetic beds be 
thick or thin, there is no hard boundary above or below them ; 
in all localities they graduate imperceptibly upwards into the Lias 
and downwards into the Keuper Marls; furthermore, the latter, 
in the co. Tyrone, merge into the Permian,* but near Belfast, 
into the Bunter Sandstone, while the relations between the 
latter and the underlying Permian is not so manifest. 
Notwithstanding these regular and continuous sequences we 
are asked to believe that during intervals of greater or less dura- 
tion, strata ceased to accumulate in this area during the period 
of time when the rocks, not represented in this country, were 
being deposited ; and that, as the Irish Permians of Belfast and 
Tyrone are equivalents of the Middle Permians, there must be a 
hiatus above them answering to the period during which the 
Upper Permians were accumulating elsewhere ; and that in the 
co. Tyrone there must be a much greater hiatus left by the 
absence, not only of the Upper Permian, but also of the Bunter 
Sandstone, and so in various other cases. 
In this way the thickness of formations is too often calculated. 
The geologist does not carefully and separately work out each 
basin of deposition, and endeavour sufiiciently to make out by 
comparison what beds or system of beds are the representatives 
and equivalents of those found in adjoining or distant basins, but 
he tries to fit in all together. He takes a bed or system of beds from 
one basin and puts them into another; then the rocks of this basin 
with those added have a place found for them in, above, or below 
the rocks of another basin, until at length a magnificent structure 
is erected—very imposing and ingenious—but without sure 
foundation, and inconsistent with what we may conclude from 
the operations which we can see going on around us at the 
present moment. Thus, formations are made to be of much 
greater thicknesses than.what they can have really attained to. 
Unfortunately for Geology there are difficulties in the way of 
its becoming a true science. First, there is the necessarily great 
room for conjecture which leaves such a vast field open for 
sensationalists and speculators ; and secondly, as in other branches 
of science, some investigators are apt to be so strongly taken 
* The limestones at Templereagh, co. Tyrone, which contain Permian fossils, are said 
to have been interstratified with the Keuper Marls. 
