250 J. E. Marr — Cambrian and Silurian Rocks. 



geologists at all agreed as to their nomenclature ; for altliongli they 

 use the terms Silurian and Cambrian, they are used in various senses. 

 In Sweden, for instance, the Lobiferus and Retiolites Beds, respec- 

 tively rejDresenting the May Hill and part of the Wenlock of Britain 

 (of. Tornqvist, Ofversigt af K. Vetensk. - Akad. Forhandl. 1879, 

 No. 2, p. 72), are by some authors classed as Lower Silurian. What 

 nomenclature then are we to adopt? No classification depending on 

 natural breaks is applicable over an enormously large area, for very 

 obvious reasons, but yet the British names of other formations than 

 the Cambrian and Silurian are very largely used by European and 

 other geologists, when describing their own areas. Nor are these 

 groupings comparable with one another as regards size ; for example, 

 the Pliocene cannot be compared in magnitude with the Carboniferous, 

 and yet these names are used as of equal value in the lists of systems 

 given in our text-books. In short, our present classification may be 

 described as an historical one; such being the case, the names Cambrian 

 and Silurian should be used in their historical sense, until our entire 

 nomenclature is I'e-modelled, and hence the word Cambrian should 

 include the rocks from the base of the Harlech Beds to the top of 

 the Bala group, as defined by its historian Sedgwick. The term 

 thus used is also natural to as great an extent as, if not greater than, 

 that for any other system, for it seems that the movements which 

 affected the northern hemisphere in the Old World were ver}'^ wide- 

 spread during the deposition of the old rocks ; the beds deposited in 

 deeper water can be traced continuously over very large areas, e.g. 

 the Arenig beds occurring as black muds in Britain, Southern 

 Sweden, France, Spain, and Bohemia, also the representatives of 

 the Birkhill Shales and of the beds characterized by Retiolites 

 Geinitzianus ; consequently if these areas were contemporaneously 

 submerged to some depth, it follows that the intermediate upheavals 

 were also contemporaneous, and the greatest upheaval throughout 

 Europe seems to have been after the end of the Bala period, so that 

 even where the deposits were not actually raised above the water, 

 as they were in many cases, there was deposition in ver}'' shallow 

 water. This is the greatest upheaval in the European area which 

 occurred throughout the periods between the deposition of the Harlech 

 Beds and that of the Ludlow, and it is accompanied by a palseonto- 

 logical break. Other physical breaks are local, and seem to have 

 beea due, not to widespread upheavals, but to local volcanic action, 

 and hence are of no classificatory value; such is the break at the 

 base of the Coniston Limestone of the English Lake District, and 

 that at the base of Etage D. of Bohemia. 



English geologists are requested to bring their nomenclature 

 before the International Comn)ission for the Unification of Geological 

 Nomenclature. A classification must therefore be adopted for our 

 Lower Palaeozoic rocks ; let us then be consistent, and add the names 

 Cambrian and Silurian in their historical sense to the remainder of 

 our historical names, especially as by so doing we make the nearest 

 possible approach to a natural nomenclature, and although late, do 

 justice to the great work of one of our greatest geologists. 



