Henry Hichs — On the Northern Falceozoic RocTcs, 159 



to go on uninterruptedly, and the sediments show perfect conform- 

 ability there at this point. Even in the British area, the breaks 

 were restricted to narrow limits, and beyond these limits the sedi- 

 ments accumulated regularly, and each epoch is there fully repre- 

 sented. An unconformity over a small area is no evidence, there- 

 fore, of a want of continuity in the greater changes that were taking 

 place, and its presence is no criterion that there was at that time a 

 general arrest in the subsidence, and hence in the contraction of the 

 crust throughout the larger part of the Northern Hemisphere. In 

 the classification of the rocks, such a line, however, is useful, and as 

 it can be recognized in more than one district in Europe, and in 

 North America, it forms a good boundary-line between the Lower 

 and Upper Silurian. The line of demarcation has hitherto been 

 placed by authors in very various positions and generally in accord- 

 ance with what has been observed by them in the places under their 

 observation. 



For present purposes I propose to retain the line adopted by the 

 late Sir Charles Lyell,^ and hence to include the Lower Llandovery 

 in the Upper Silurian. Another reason for doing this is the tolerable 

 universality over the British area of a band of limestone at the close 

 of the Bala period, and which seems to have been formed just before 

 the uplifting took place. It is known in parts of Wales as the Hirnant 

 Limestone, in Shropshire as the Caradoc Limestone, in Cumberland 

 as the Coniston Limestone, and in Scotland as the Wrae Limestone, 

 so we have here a good line formed, and over considerable areas. 

 In some of the undisturbed regions in Europe and in America the 

 formation of limestone which commenced early in the Lower 

 Silurian continued to go on iminterruptedly at its close, and there 

 bridged over and connected it with the Upper Silurian so closely that 

 no doubt can arise as to the continuation of similar general con- 

 ditions throughout the greater part of these two epochs in those 

 areas. In Britain limestone beds of any importance are first found 

 in the Llandeilo Epoch, and were it not for the frequent spreading 

 out of the volcanic material over the sea-bottom, there can be no 

 doubt that the Lower Silurian period, like that in America, would 

 there be represented chiefly by limestone formations. At present 

 they occur at invervals in the series, and they show evidence of 

 having been deposited only during quiet periods intervening between 

 the more active eruptions. These limestones were doubtless formed 

 entirely by marine life, and in many places they are now exceedingly 

 rich in fossils. A quiet. period consequently would be required for 

 the building up of any considerable thickness, and as they are now 

 thickest the farther removed they are from the disturbed areas, it is 

 evident that the interruptions were due to the life being suddenly 

 destroyed or checked, and from being covered over also by an excess 

 of material thrown out over the sea-bottom. It is probable also that 

 where some of these formations were built up, a barrier in the shape 



^ Prof. Hughes also recognized this line as being preferable to the one at the base 

 of the Upper Llandovery, and makes this the boundary between Cambrian and 

 Silurian. 



