SS (| 1 | Mr J. EF. Marr, On Homotacais. 81 
of correlation. It must be remembered however that they are not 
dominant forms, and that there is no difficulty in distinguishing d, 
from d, and so with the other bands. Moreover it is not even 
necessary to imagine migration from the area during the period of 
accumulation of the intervening sediment, though very probably 
this did occur. We do find some forms which are abundant in two 
beds and which are very rare in the intervening ones, and it is 
possible that in the cases above quoted, the forms struggled on in 
the area under unsuitable conditions, and again increased in 
numbers when the conditions became once more favourable. In 
this case they might well escape detection in the intervening beds, 
even when the beds were so thoroughly worked as were those 
of the Bohemian basin by Barrande. ; 
F. Possible cause of the change of graptolitic faunas. It has 
been seen that change of lithological character does not necessarily 
produce change of fauna, though the two do frequently accompany 
one another, and therefore seem to be produced by some common 
cause. This cause is either physiological or physical. The co- 
incidence of a complete change of facies of the organisms with that 
of the containing rocks seems to point to the latter. Yet we find 
in the graptolitic bands, that thin pale green bands occur which 
are identical in lithological character with the black graptolitic 
bands, save for the colour and the great rarity of graptolites con- 
tained in them. The colouring matter of the black bands is 
certainly organic, and seems to be due to the graptolites them- 
selves. Yet these pale green bands are very constant. One of 
them, a quarter of an inch thick, occurs at precisely the same 
horizon in the Lake District for a distance of over half-a-dozen 
miles, and is not separated from the black graptolitic bands above 
and below by any plane of stratification. As it appears that this 
deposit is quite like the rock in which it lies, except for the absence 
of graptolites, we must look for some cause which would cause the 
graptolites to disappear from the area, or to linger on under great 
disadvantages, without altering the character of the sediment. 
The only cause I can think of is change of climate; I suggest this 
as a possible explanation, without insisting on its probability. If 
such climatic changes occurred, and are accountable as Dr Croll 
thinks for variations in the character of Carboniferous deposits, 
they are equally likely to have produced similar effects here. In 
this case, all the difficulties of synchronism are overcome, and we 
see a reason for believing what the evidence certainly supports, 
viz.: the possibility of successfully correlating deposits of remote 
areas. 
G. Conclusion. An examination of the facts seems to me to 
indicate that we need not be frightened of attempting correlations 
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