130 NATURAL SCIENCE. ^rRiL. 



Another use which can be made of the method we are studying 

 is the more accurate estimate of the physical conditions under which 

 groups of strata were deposited. This kind of work has hitherto 

 been carried on in a somewhat haphazard way as far as the Lower 

 Palaeozoic Rocks are concerned, but as an example of the results 

 which we may eventually get by careful study of zones, I may point 

 to Professor Lapworth's admirable section showing the lateral 

 changes observable in the Ordovician and Silurian strata of the 

 Southern Uplands, appended to his paper on the Ballantrae Rocks, 

 which appeared in the Geological Magazine for 1889. 



The study of zones not only throws light upon the physical 

 conditions under which they were deposited, but also gives us 

 additional evidence of the value of these zones. The expansion of 

 thin masses of shale into much thicker deposits elsewhere, and the 

 swarms of fossils occurring in those deposits, has been held for some 

 time to indicate that the thin sets of strata were formed far from 

 land, and this is supported by observation of the nature of the 

 included fossils. It has been shown by Dr. Hinde that deposits of 

 radiolarian chert are associated with the graptolitic shales of Scot- 

 land, and these have been compared with similar modern deposits 

 formed in deep sea. Professor Suess has given instances of trilobites 

 amongst the Lower Palaeozoic Rocks which strikingly resemble 

 modern deep-sea crustaceans in more than one respect, and such 

 trilobites are found amongst the strata which are divisible into thin 

 zones. It would seem, therefore, that the evidence points to such 

 rocks having been formed in seas far away from land, and in that 

 case we need not be surprised at the very wide geographical 

 distribution of the organisms of the zones. 



The in(|uiry into the cause of zones may possibly throw some 

 light on climatic change, though here we are touching upon a ques- 

 tion upon which it would be premature to speculate. Suffice it to 

 say, that the sharp line of demarcation between many zones does not 

 appear to be due to change in the physiographical conditions of the 

 area (apart from climatic change), for we find sediment having the 

 same lithological characters, divisible into several distinct life-zones, 

 and, as already stated, the same zone may be traced among strata of 

 very different lithological characters. 



In studying the phylogeny of different groups of organisms, the 

 establishment of zones will prove invaluable. Numerous errors have 

 shown the danger of attempting such a phylogeny by a study of 

 fossils in the Museum, without reference to their order of appearance 

 in the strata. On the other hand, an earnest of what may eventually 

 be done is furnished by the contribution to the study of the descent 

 of the later graptolites given by O. Hermann in his monograph of 

 the Dichograptidae. 



The zonal method has " quietly entered upon what may be 

 regarded as the accepted or orthodox stage " — so wrote Professor 



