Distribution of the Rhabdoplwra. 203 



question of time ; and in these zones the geologist is 

 presented with a new and invaluable key to the elucidation 

 of the details of the succession among the Lower Palaeozoic 

 rocks — a key of far wider application than any formerly at his 

 command. The fossil Crustacea and Brachiopoda, on which 

 he has hitherto been content to rely, are, as a general rule, con- 

 fined to sediments containing a large proportion of carbonate 

 of lime. The limestones and calcareous sandstones in which 

 they occur most abundantly are usually so diversified in their 

 petrographical characters that the working geologist is often 

 wholly able to dispense with the aid of the palaeontologist in 

 determining the limits and inter-relationships of their contain- 

 ing strata. But these highly calcareous deposits constitute 

 merely an insignificant fraction of our Lower Palaeozoic sedi- 

 ments. The vast majority are grits, flagstones, and shales, 

 containing a most minute proportion of calcareous matter, and 

 from which, as a consequence, the lime-loving forms are wanting. 

 These enormous accumulations of strata, composed of endless 

 repetitions of similar rocks, incapable of subdivision by petro- 

 graphical characters, and destitute of the special fossils upon 

 which the palaeontologist relies for guidance, the geologist 

 has hitherto been compelled to leave undivided. The unbroken 

 sheets of the flag-like Silurian rocks of Hereford, Merioneth, 

 and Denbigh, which are shown upon the Survey maps in two 

 doubtful divisions only, as contrasted with their minutely sub- 

 divided prototypes of Siluria, afford us a case in point. Other 

 examples are seen in the wide-spreading sheets of Ordovician 

 and Silurian strata which, unbroken by a divisional line, 

 cover many thousands of square miles in Middle and North 

 Wales, South Scotland, and Ireland. Now in these mono- 

 tonous strata, so barren of organic remains of the higher 

 groups, the lowly Graptolite is a frequent and characteristic 

 fossil; and by its aid the geologist of the future will be able 

 to read off the natural succession among these undivided sedi- 

 ments with ease and certainty. 



(ix.) The several zones common to two or more regions 

 occupy invariably the same relative position with respect to 

 each other, and the same vertical place in the ascending series 

 of formations. Hence we have no choice but to regard them 

 as homotaxially or synchronologically identical. It will be 

 seen from Table XII. that, as a general rule, the zones are 

 not recognizable scattered irregularly over the globe, but that 

 they occur more or less in groups, being restricted in their 

 range to neighbouring geographical regions. Hence it is 

 highly probable that we see in many of these zones the relics 

 of what were originally special subformations or stages, once 



