STRATIGRAPHICAL GEOLOGY 



South Wales. North Wales. Scotland. 



Carboniferous. Carboniferous. Carboniferous. 



Upper O.R.S. Upper O.R.S. 

 Middle O.R.S. 



Lower O.R.S. Lower O.R.S. 



Silurian. Silurian. Silurian. 



Ordovician. Ordovician. Ordovician. 



Cambrian. Cambrian. Cambrian. 



Thus, if we adopted the arrangement warranted by the strati- 

 graphical breaks which occur in South Wales, we should have two 

 separate systems, and two only, for there is a general conformity 

 between the Cambrian and Ordovician, and from the Silurian to 

 the Carboniferous. 



In North Wales, however, the break between the Ordovician 

 and Silurian is much less marked, while there is certainly one 

 above the Cambrian, and there is an entire absence of Old lied 

 Sandstone, so that we should make three separate systems. Again 

 if we pass to the south of Scotland we find complete conformity 

 between the Ordovician and Silurian, and no marked break 

 between the latter and the Old Eed Sandstone, but a great 

 one between the two divisions of this Eed Sandstone formation, 

 lieiice we might also make three systems in Scotland, but they 

 would be quite different from those in Wales and would be of very 

 unequal value. 



As in this case, so also in others, a classification founded on 

 physical breaks may be conveniently used for a limited district, 

 and until further knowledge is acquired ; but the breaks in the 

 succession are certain to be filled up elsewhere by transitional beds, 

 and as these may be of systematic value, we must always be 

 prepared for the possible intercalation of another system between 

 any two groups that are divided by a decided unconformity. 1 



In brief, it must be admitted that unconformities are geological 

 accidents, and if our classification and nomenclature are to be 

 applicable to any large section of the earth's crust, they must be 

 based, not on stratigraphical but on palaeontological facts. The 

 Ordovician, Devonian, and Jurassic are systems so established ; and, 

 in process of time, as our knowledge becomes more and more 

 complete, it is probable that the number of systems will be finally 

 determined in the same manner. They will always be more or less 

 arbitrary divisions, since it will be difficult to say what number of 



1 The student can read with advantage Professor Lapworth's article on 

 the " Classification of the Lower Palaeozoic Rocks " in the Geological Magazine, 

 Dec. 2, vol. vi. p. 1 (1879). 



