1887.] Mr J. EF. Marr, On Homotazis. . Cis 
was in existence, which is more probable. It has been observed 
furthermore, that a similar resemblance may be traced with 
species, even in the case of thin zones of rock, and the alternatives 
are the same in this case also. 
Secondly, although the change of faunas is not necessarily 
accompanied by a complete change in the lithological characters 
of the rocks containing them, it is frequently so marked, and even 
if not, is very often accompanied by a change of what I have 
spoken of as the accidental characters of the rocks. This is also 
traceable to some extent, in comparing thin zones, as I hope to 
shew elsewhere. If the change of lithological character does not 
produce the change of organisms, and I have given reasons for 
supposing that it does not, we are compelled to conclude that the 
organic and lithological changes took place simultaneously in one 
area, and (according to the Homotaxis theory) later on occurred 
also simultaneously in another. In any case it points to the con- 
clusion that lithological and organic change were due to one and 
the same cause, which cause may have acted simultaneously over 
wide areas, nevertheless, one objection which has been made to the 
correlation of deposits as synchronous, is founded upon the migra- 
tion of faunas, owing to lithological change. 
D. Possibility of ascertaining practical contemporaneity of beds 
over wide areas. We have been driven to admit either that 
eroups of species originated after each other in one and the same 
area, and migrated thence along the same lines time after time, or 
else that the time taken for dispersal was short, as compared with 
the time of the duration of each group of species. But as in some 
cases, the groups of species are limited to zones a foot or two 
in thickness, the time taken for dispersal was short as compared 
with the time necessary for the accumulation of the sediment of 
these zones. In other words, the amount of sediment accumulated 
in these cases. during the period of dispersal of the organisms was 
so thin, that it may be practically neglected, and the zones spoken 
of as contemporaneous, Thus though we admit that a film of 
sediment may have been laid down on the sea-floor of the 8. Scotch 
area during the formation of the rocks of the gregarius-zone of 
the Birkhill shales, before it was accumulated in 8S. Sweden, it is 
hard to resist the conclusion that these two zones were being 
formed simultaneously in the two areas during most of the period 
when their characteristic organisms existed. If this be true of 
these minor subdivisions, it is much more true of the major ones, 
of which they form but a fraction. 
EL. Difficulties in the way of admitting synchronism of formation. 
It may be objected that the cases upon which I have laid most 
stress are cases of exceptional slowness of accumulation of sediment, 
VOL. VI. PT. TI. 6 
