226 J. G. Goodchild—On “ Overlap,’’ ete. 
places, beyond all the Lower Carboniferous strata, so as to lie directly 
upon the older rocks. Another illustration is supplied by the moun- 
tain limestone between the south-eastern border of Westmorland and 
the district of Craven, which, as it is traced from the neighbourhood 
of Sedbergh, where the lower members of the formation are very 
fully developed, can be shown to overlap what is practically the 
whole of the vast thickness of the Upper Old Red, as well as some 
of its own lower strata, and to come down into direct natural contact 
with the Pre-Carboniferous rocks as we advance in the direction of 
the Craven Faults. To convey a correct idea of this kind of relation 
of one member of a series to another, it would probably be found 
difficult to employ any more expressive, and, at the same time, more 
suitable term than that of Overlap. 
But it is not always convenient, or possible, to take as the starting- 
point in a description that part of the district where the downward 
development of the formation under notice attains its fullest known 
dimensions. In such a case it might be advantageous to add to our 
descriptive terminology by employing some other word correlative 
in its meaning with overlap. J would here suggest as a term likely 
to be of service for such a purpose the corresponding term Underlap. 
The examples above given may be again cited in illustration of its 
use. I should say that as the Coal-measure strata referred to on the 
borders of Wales are followed in the direction where deeper-water 
conditions prevailed in the Carboniferous Period, we find first the 
stratigraphical equivalents of the Millstone Grit, and then of the 
Lower Carboniferous subformations successively wnderlapping the 
strata that, nearer the centre of the Principality, he directly upon 
the rock of the various Pre-Carboniferous series. Or, to take a case 
in the North of England, still more to the point for the present pur- 
pose, I should say that the Roman Fell beds of the Pennine Escarp- 
ment right and left of Milburn, as they trend on the one hand 
towards Gamasby, and on the other towards Brough, are gradually 
underlapped by the Upper Old Red, in just the same way as the 
Welsh Coal-measures are underlapped by the Lower Carboniferous. 
Overlap, and its correlative Underlap, as it is here suggested to 
employ them, thus denote the stratigraphical relations of one part of 
a formation to another where the original extent of the lower part 
occupied a smaller area than that of the next higher members of the 
same series. 
If it should, on consideration, hereafter be deemed advisable to 
re-adjust the signification of the term Overlap to the extent here 
suggested, a vacancy in our terminology will be created, and another 
term of parallel meaning will have to be provided to denote the 
corresponding phenomena in the case of true unconformability. 
It is by no means an uncommon case to find two groups of 
associated strata presenting examples of both overlap and uncon- 
formity in one and the same section. On the large scale the Coal- 
measures of the Welsh Border both overlap the Lower Carboniferous 
strata, and extend transgressively across the edges of the rocks older 
in the geological series. The New Red of Leicestershire behaves in 
