J. G. Goodchild—On “ Overlap,” ete. 220 
making a useful addition to our stock of descriptive terms by allowing 
it to revert to its original meaning. This principle is generally 
admitted to be a good one; nevertheless, in practice it is by no 
means always observed. Numerous as are the terms already at our 
disposal, the advance of knowledge is constantly reminding us of 
the necessity of multiplying rather than of lessening the number 
of terms of specific signification as occasion requires us to limit the 
meaning of related terms already in use. 
As an instance of a word whose employment commonly gives rise 
to ambiguity may be mentioned the word Overlap. For example, 
when a writer has occasion to make the statement that a certain 
stratum “ overlaps” another, it is often impossible, without a refer- 
‘ence to the context, to discover whether the case referred to is that of a 
higher member of a continuous series of deposits coming into natural 
contact with a lower, as a consequence of the local attenuation of a 
third stratum elsewhere known to occur between them ; or whether 
it refers to a case of true unconformity, where there is no necessary 
sequential relation between the bed that “overlaps” and the rock it 
may happen to lie upon. The two cases are, of course, essentially 
different. It is true that in the first case mentioned the contact of 
the upper stratum with the lower can be shown to be due to a kind 
of break in the succession, inasmuch as sufficient time must have 
elapsed for the accumulation of the stratum that is elsewere known 
to intervene. But the group as a whole may have been deposited 
from beginning to end with no more important interruptions than are 
incident to the accumulation of any sedimentary deposits of the 
ordinary kind. The case is simply one of wedge-bedding of greater 
horizontal extent than usual. 
Or the kind of overlap referred to may be a case of essentially the 
same nature as the last, manifested under a form slightly different ; 
as, for example, where the higher bed referred to extends beyond the 
lower as a consequence of the deposition of horizontal strata upon a 
surface that is itself inclined. A wall built up the slope of a hill 
supplies a familiar illustration of a case of this kind. If the 
masonry is laid horizontally, then course after course, from the lowest 
upwards, extends in succession beyond the others, with the result 
that a particular line of masonry may at one spot form the very top 
of the wall, and yet be, at the end next the hill, in actual contact 
with the foundation. 
This last case may be regarded asa typical example of what several 
authors have meant by the word overlap. But as, in nature, 
the several strata are usually found to thin away where their edges 
approach the bank, instead of ending off against it abruptly as 
the masonry does, it is perhaps hardly advisable to advocate the 
introduction of any distinction between the terms employed to denote 
the approximation of two beds as a consequence of the lenticular 
deposition of a third, and the unequal extension of strata deposited 
upon an irregular surface. 
As well-known illustrations of typical overlap I would refer to 
the Coal-measure outliers on the borders of Wales, which extend, in 
DECADE II.—VYOL. X.—NO.: V. 15 
