J. G. Goodchild—On “ Overlap,” ete. 227 
precisely the same way, so does that of Kirkby Stephen, in West- 
morland; or, again, the Mountain Limestone between Sedbergh 
and the Craven Faults, as already referred to. It is not often that 
any such striking examples as the last mentioned can be cited, 
especially where the rocks under description appertain to the higher 
horizons of the Neozoic division. But among the Palaeozoic rocks 
the perfect distinctness of the two phenomena hitherto included 
under the one term Overlap is constantly forced upon the attention 
of the field-geologist. For example, the quartz-conglomerates of the 
Roman Fell Series in the Pennine area, overlap the perfectly-distinct 
conglomerates of the Upper Old Red; while the Carboniferous rocks 
as a whole (assuming that the Upper Old Red really forms a conform- 
able subformation of the Carboniferous series) are known to extend, 
within ‘a short distance, across the denuded edges of strata ranging 
from the Skiddaw Slate up to the Coniston Grit at Gamasby. In 
other words, while one of the minor subdivisions can be shown to 
overlap another subdivision of the same series to the extent of a few 
hundred feet or so, that series as a whole is transgressive across the 
edges of the older rocks to an extent that may be stated in miles. 
In such a case the necessity of employing separate terms to enable 
us clearly to convey the distinction observable in the field between 
the one kind of “overlap” and the other will at once be recognized. 
The question arises, what term is there that is not already appro- 
priated and whose suitability to the purpose in view would be likely 
to be generally admitted ? 
Diagram to illustrate the distinction in meaning between the terms Overlap and 
Overstep, and their respective correlatives Underlap and Understep. In the 
figure, beds 1 and 2 of series A successively overlap bed 3, while the series 
A, as a whole, oversteps the various subdivisions of the older group B. 
Conversely, 3 underlaps 2, as this itself underlaps 1; and the several members 
composing the series B understep the formation that lies unconformably across 
their edges. 
In oral descriptions I have found it convenient, in explaining these 
points upon the ground to my colleagues and others, to speak of this 
stratigraphical relation of unconformable beds to the various rocks 
immediately beneath as Overstepping. For example, I should say 
that the Roman Fell Beds in the neighbourhood of Melmerby over- 
lap the Upper Old Red, while the Carboniferous formation—whose 
basement beds are formed by the Upper Old Red and the Roman 
