80 Mr J. E. Marr, On Homotazis. [Feb. 14, 
and such is indeed the case. For example, Prof. Lapworth has 
shewn that the 140 feet of Birkhill shales of the Moffat district 
are represented in the Girvan area by over 1000 feet of sediment, 
(Q. J. G. S., Vol. xxxvin. p. 537). The result of this thickenmg 
out is not to confuse the zones, but to cause the fossils to be 
distributed through a greater thickness of beds. There is no 
practical difficulty raised in the case of sediments of greater 
thickness than those we have specially considered. The effect will 
be to cause the geologist to subdivide the rocks into zones of 
greater thickness, rather than to make him attempt to speak of 
deposits as synchronous which are in reality not so. It is found 
also in actual fact, that the quickly formed deposits are those 
which rapidly change their characters when traced laterally, 
whereas the slowly-formed ones have a wider distribution without 
much alteration. In other words the slowly-accumulated deposits 
were wide-spread horizontally, the quickly-formed ones were local. 
This is the case at the present day. In attempting to correlate 
the rapidly-accumulated deposits of widely distant areas, one meets 
not only with changes of lithological character, but also with 
differences in the faunas, so that there is no temptation to insist 
upon the synchronous formation of different portions of these 
deposits. I may make this clearer by an example. Suppose we 
have in two widely separated areas, thin deposits of A age divisible 
into zones a, b, c..., and these zones are similar in the two areas. 
Above these deposits we meet with thick deposits of B age, which 
are mainly grits in one area and calcareous ashes in the other, and 
that these are succeeded by deposits of C’ age in the two areas, 
which as in the case of those of A age are divisible into zones 
L,Y, 2.... Which are similar m the two areas: then, although we 
have proof that the B group are really as a whole contemporaneous, 
we shall be unable to compare their subdivisions to any extent, for 
we shall find a succession of zones of sand-loving organisms 1, 2, 3 
in the one case, and of another set 1’, 2’, 3’ having a different 
habitat in the other. 
The case of reappearances must be considered. It is well known 
that fossils occur in certain bands of the Bohemian Lower Palzeo- 
zoic rocks, which appear to be absent from intermediate bands. 
Barrande divides his stage D into 5 bands, of which d,, d,, d, have 
a general resemblance to one another, and differ from d, and d, 
which are in turn somewhat alike. In d,, d, and d, we find 
fAiglina rediviva and Dionide formosa, whilst the genera Areia, 
Carmon, Lichas, Ogygia, and Proetus occur in d, and d, and not in 
the mtermediate bands. In d, and d, are Illenus distinctus and 
I. transfuga, which are absent from the other bands. Barrande 
shews a similar set of recurrences of fossils from g, in g, and of g, 
in h,. These examples seem to throw doubt upon the possibility 
