,g,,. LIFE-ZONES IN LOWER PALEOZOIC ROCKS. 127 



group of deposits is studied, there is a tendency to speak rather of 

 series or stages than of zones. Nevertheless, the term is not applied 

 merely to thin deposits, but also to groups of considerable thickness, 

 when a particular form is characteristic of a group of strata. 

 Professor Lapworth, when commenting upon the thinness of the 

 Stockdale shales (4) (divisible into nine graptolitic zones), pointed out 

 " that they were represented by very great thicknesses of deposit 

 elsewhere ; thus, the Browgills were represented by thousands of 

 feet in the Bala group and the Tarannon, and the Skelgills by 

 enormous thicknesses in Girvan and Central Wales." The various 

 zones of Llandovery-Tarannon age are traceable also in these 

 expanded deposits, and there each zone is of very considerable 

 thickness. I have elsewhere spoken of the Lower Ludlow Rocks of 

 Bohemia as the zone of Monogyaptus colomis, and there is no reason 

 why the term should not be applied to those beds, though, doubtless, 

 a more intimate acquaintance would show that they might be split 

 into smaller zones or sub-zones. The beds of this age in Bohemia 

 are not very much thicker than some of the recognised graptolite 

 zones of typical areas in Britain ; but in the English Lake district 

 we find that these Lower Ludlow Rocks {i.e., the zone of Monogmptus 

 colomis) have an estimated thickness of about 10,000 feet, and although 

 this may be an exaggeration, there is no doubt that they must be put 

 down at many thousands of feet. 



The above remarks lead us to consider the lateral extension of 

 zones as hitherto traced. Using the term zone in the widest sense 

 in which it has been applied, we find zones characterised by a par- 

 ticular genus having a very extended geographical distribution. 

 The zone of Dictyogvaptus has been recognised in Britain, Belgium, 

 Russia, Scandinavia, and America ; that of Bvyograptus in Britain, 

 Scandinavia, and America, and that of Phyllogvaptus in these three 

 countries, as well as in Australia. There seems no reason why 

 strata characterised by particular genera should not be spoken of as 

 zones, and indeed we have long been accustomed to speak of the zone 

 of Micrasters, and, more recently, of the Olenellus zone. At present, 

 however, we may confine our attention to such zones as are 

 characterised by the existence of forms which are usually spoken of 

 as species. These zones vary in different ways, when traced 

 laterally. It has already been pointed out that they frequently 

 expand considerably when followed from one area to another, and, 

 on the other hand, they thin out to such an extent as sometimes to 

 disappear altogether. Thus the Rastriies maximiis zone of the Stock- 

 dale shales is found near Sedbergh, but does not occur in the Lake 

 district, and study of the deposits points to the conclusion that it has 

 actually thinned out, and is not represented by any sediment. In 

 such a case the absence of sediment in an area during the time that 

 the deposits of a zone are being laid down elsewhere, will result in 

 the production of a plane of stratification, marking the position of 



