Correspondence—Mr. A. J. Jukes Browne. 009 
Student’s Manual and Green’s Physical Geology), except that he 
omits all mention of its necessary connection with unconformity and 
even appears to suppose that overlap may take place without any 
concomitant unconformity. I would ask Mr. Goodchild whether he 
could draw a case of overlap (in his sense) without an unconformity 
existing at the base of the series in which the overlap occurs; the 
case drawn in his figure (p. 227) is an ordinary one with an uncon- 
formity. So far it seems to me that he has only introduced more 
confusion into the subject than there was before. 
His final suggestion is, however, much more to the purpose, and 
we now come to the point where a confusion does really exist in the 
minds of geologists. I think it is ordinarily supposed that trans- 
gressive and overlapping are convertible terms, but are they ? and 
are not Mr. Goodchild’s remarks really directed against the confusion 
which has arisen, from the want of a proper distinction between 
them ? 
If overlap be ‘correctly defined as a relation between two conform- 
able groups of strata, and as consisting in the extension of the 
higher group beyond the limits of the lower group so as to rest upon 
some member of an older series, as shown in Mr. Goodchild’s 
diagram ; then it is clear that the same term should not be applied 
to a relation between unconformable strata, such as the transgression 
of a single stratum across the edges of groups belonging to an older 
series. ‘This relation is indicated in the accompanying diagram, but 
would be better shown in a plan, in which the outcrops of the 
groups d, ¢, b, are gradually and successively hidden by the trans- 
gression of the group m across the edges of their component beds. 
This is a very different phenomenon from true overlap, and yet the 
so-called overlap of the Chalk in Yorkshire is exactly a case of this 
kind; the Red Chalk there is continuous and is not overlapped by 
anything, but is itself transgressive across the different members of 
the Jurassic series. 
The difference in the nature of these two relations has, I suppose, 
been partly perceived by those who would speak of a conformable as 
opposed to an unconformable overlap, but such a distinction does not 
avoid confusion, while it introduces a cumbrous terminology, and 
I quite agree with Mr. Goodchild that, since the two things are 
essentially different, it only perpetuates confusion if we apply the 
same name to both. The only question is whether there is any 
necessity to invent a new term and whether that already in 
use, viz. transgression, is not sufficient for the purpose, so long as 
authors are careful to make the necessary distinction between over- 
lap and transgression. The latter term has, however, acquired 
another meaning in our language, and I am therefore inclined to 
