128 NATURAL SCIENCE. alr,,.. 



the zone. According to Professor Lapworth (5), the fossiliferous zones 

 of the Moffat series " disappear one by one from above as we pass 

 over the Uplands from south-east to north-west." Furthermore, the 

 sediments of a zone may vary considerably when traced laterally, 

 whilst the fossils remain the same. The graptolites of the Lower 

 Ludlow beds are similar in the limestones of Bohemia, the shales 

 of Scandinavia, and the grits of the north of England. At other 

 times, both the lithological characters and the organic contents of 

 corresponding strata are different in two areas. In such cases, as a 

 rule, only general comparisons can be made, and a minute zonal 

 subdivision of the strata cannot be attempted. 



The most rigid test to apply to the zonal method, if we wish to 

 ascertain its value, is the examination of the fossils of zones, with a 

 view to discovering whether there is not an inversion of the order of 

 some of the zones in different areas. So far as I am aware, the 

 extensive exploration that has been carried on amongst the Lower 

 Palaeozoic Rocks has hitherto brought to light no case of the inversion 

 of order of two zones in any two areas. It would be tedious to give 

 instances of the constancy of order of such zones, but they may be 

 readily found by searching the papers which, during the last fifteen 

 years, have been devoted to the distribution of the graptolites. It 

 is difficult to see how, on any view save that of the contemporaneity 

 of the strata, marked by the inclusion of the same fossils, we are to 

 account for this absence of inversion. It might be produced by the 

 successive zone-forms originating in one, and only one, area, and 

 spreading away from thence ; but such a supposition, though perhaps 

 just conceivable, is in the highest degree improbable, and viewing 

 all the evidence, not of the graptolites only, but of other forms also, 

 may be rejected. But all difficulties vanish if we allow sufficient 

 time for the accumulation of the strata. Surely the time taken for 

 the spread of an organism by transport or migration would usually be 

 short, as compared with the time during which that particular form 

 existed. Where a species is limited to a few feet of strata, the time 

 during which that species spread may be represented by less than an 

 inch of deposit. In this case, the zones, though not actually begun 

 and ended at the same time, are for all practical purposes contempo- 

 raneous. We should speak of a man who was born in 1800, and 

 who died in 1870, as tlie contemporary of one who was ushered into 

 the world in 1801, and left it in 1871 ; and in a science like geology, 

 where accurate estimates of time are at present impossible, and seem 

 likely always to remain so, a similar use of language is fully 

 justifiable. If the value of zones depends on the comparative 

 rapidity of dispersal of organisms, a group of organisms should be of 

 more or less value according to its capacity for rapid dispersal ; and 

 if this be so, some forms should be more useful than others in 

 marking zones. It is very probable that such freely floating 

 creatures as graptolites were more easily dispersed from their centres 



