Ordnance Geological Survey of England. 517 
Wales can be as accurately laid down upon a map as the parts of 
South Wales to which I will presently allude; though when the 
operations of the Ordnance surveyors are extended to this com- 
plicated region, we shall learn, by distinct geometrical admeasure- 
ment, the exact thickness of these suberystalline rocks on the flanks 
of Snowdon. 
Mr. Murchison concludes this part of the subject, by expressing his dis- 
sent from a proposal made by Mr, Sharpe to strike out the “ Llandeilo 
Flags ” from British nomenclature. 
Ordnance Geological Survey of England.—The progress which 
was confidently expected at the hands of the Ordnance Geological 
Survey, directed by Sir Henry De la Beche, has recently been so 
effectively extended to a country with great part of which I am well 
acquainted, that, whilst we are considering the subject of the Pa- 
lezozoic Rocks, I take the opportunity to add my tribute to the large 
share of public approbation which such labours must earn for their 
authors. If my few comments on this subject involve reference to 
my own work, I trust the Society will believe that such allusions are 
made solely to explain the subsequent progress of other geologists. 
In my last Address I alluded to the valuable researches of the 
Ordnance Geological Survey in South Wales, particularly in the 
great coal-basin; and I have now to speak of them amid the older 
rocks of Pembrokeshire and Caermarthenshire, forming the south- 
western tracts of the country termed the Silurian region. In the 
survey of that region, my chief object, as you know, Gentlemen, 
was simply to ascertain the general classification and right order of 
certain fossiliferous strata beneath the Old Red Sandstone. Having 
worked out the succession in typical districts in Shropshire, Here- 
fordshire, Radnor and Montgomeryshire, I afterwards traced them 
to the south-west, until I equally determined their relations to the 
superior deposits in the coast-sections of South Wales. Although 
the labours in the latter country were thus auxiliary only to those of 
the arena on which the classification was established, I have had 
great satisfaction in finding, that my chief boundary lines of Old 
Red Sandstone and Upper and Lower Silurian Rocks are pretty 
nearly those which have resulted from the very systematic Ordnance 
Survey, the first corrected field-sheets of which Sir H. De la Beche 
has allowed me to view. ‘This observation has reference only, how- 
ever, to the development of what may be called one zone of Silurian 
rocks, or that to which, as contiguous to the Old Red Sandstone, I 
gave my chief attention. Of the existence of true Silurian rocks 
to the west and north of a certain line which was set up as a de- 
scending limit in South Wales, I was, I confess, entirely ignorant. 
Finding no fossils in the few visits which I made to the west of that 
barrier in Caermarthenshire, which was provisionally agreed upon, 
and to the north and west of which all the country was ultimately 
to be explored by my friend Professor Sedgwick, we both of us be- 
lieved, that such tracts, for the most part without fossils, were of 
higher antiquity than the Silurian districts, and that, rising up from 
