AUSTEN EXTENSION OF THE COAL-MEASURES. 53 



lies a surface of " Old Silurian " rocks : the two are unconformable, 

 and the older had already undergone those changes which imparted 

 to it its present mineral character, at some time anterior to the newer 

 formation, which has in fact been mainly derived from it. Again, 

 the Irish area does not present that assemblage of marine forms 

 which is known as "Upper Silurian," nor yet a true "Devonian" 

 or "Eifelian" fauna; so that from these combined considerations 

 we are justified in the inferences — 1st. That the Irish area was 

 wholly included in that movement of elevation which effected the 

 olden Siluro-Cambrian sea-beds ; and, 2ndly, that it became a terres- 

 trial surface anteriorly to the oldest carboniferous depositions. 



Lacustrine formations imply subordination to areas of land : with 

 respect to the Irish area, the extension of this land was to the north 

 and west, and as in all cases the extent of the lacustrine area may be 

 taken as a fair measure of the terrestrial surface from which it was 

 supplied, that land must have had great expansion, or elevation, or 

 both. 



The hmits of the lacustrine Old Red Sandstone of the Welsh 

 area are tolerably well defined ; they hardly extended as far north as 

 ordinary geological maps now carry that group, inasmuch as the red 

 beds which underlie the coal-measures of Coalbrook Dale belong to 

 the uppermost Ludlow beds, with Lingula cornea, &c. 



In the South Staffordshire coal-district, and which m-ay be taken 

 as an extension of that of the Forest of Wyre and Coalbrook Dale, 

 the coal-measures rest immediately on marine Silurian strata ; and 

 this relative position, to the exclusion of any Old Red Sandstone, is 

 continued as far as Charnwood Forest, where the coal-measures rest 

 on some of the oldest portions of the Palaeozoic series. 



This old ridge, which is thus clearly defined as to range, if not to 

 its full former extent, was a limiting barrier to the " Old Red Sand- 

 stone," and a parting barrier to two areas of typical Mountain Lime- 

 stone ; — considerations which clearly indicate very early terrestrial 

 conditions in this direction ; or else that it was placed out of the 

 reach of the depositions of that time, from another set of considera- 

 tions, to which reference will be made in the sequel. 



There are traces of "Old Red Sandstone" at intermediate places 

 between the two great areas of Wales and Scotland ; and these are 

 of interest, as they indicate with equal clearness an immediate sub- 

 ordination to a terrestrial surface. The little patches which are 

 dotted along the eastern skirts of the mass of the old slate moun- 

 tains of Westmoreland and Cumberland are in every case so closely 

 related to the rocks of the particular locality, as to suggest that they 

 are the alluvial beds of the ancient valley-courses of that region. 

 A like local relationship, as was long agd observed by M. Bone '^, is 

 to be traced along the whole of the junction-line of the " Old Red 

 Sandstone of Scotland ; " and here the accumulation often parts 

 with its character of water-rounded conglomerate, and assumes that 

 of the angular talus so common in subaerial detritus. 



* Geol. de I'Ecosse. 



