240 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 16, 



limestones from the so-called gneiss, since it is through this latter 

 that the felstone penetrates, and not through a line separating this 

 from the quartzy or calcareous deposits." 



The changes which are involved in the adoption of my views 

 of the order of succession are, it will he admitted, considerahle. 

 In the first place, hy showing that mountain-masses of sandstone 

 and conglomerate he unconformahly beneath quartzose and cal- 

 careous rocks with true Lower Silurian fossils, we know that the 

 former must be of Cambrian age. We further learn that the old 

 or fundamental gneiss, which lies beneath such Cambrian sand- 

 stone, and is entirely unconformable to, and independent of it, is 

 a lower stratified rock than any hitherto recognized in the British 

 Isles. The beginning of the geological alphabet, as applied in the 

 Maps of the Geological Survey to the Cambrian rocks of England, 

 "Wales, and Ireland, must therefore be preceded in Scotland by the 

 first letter of some alphabet earlier than the Roman, showing a 

 still lower deep in the north-west of Scotland (as in ]N"orth Ame- 

 rica) than exists in England, Wales, or Ireland. 



If this most ancient gneiss required a British name, it might in- 

 deed with propriety be termed the " Lewisian System," seeing that 

 the large island of the Lewis is essentially composed of it, capped 

 here and there by derivative masses of Cambrian conglomerate ; 

 but the term " Laurentian " having been already applied to rocks of 

 this age in North America by our distinguished associate Sir W. Logan, 

 I adhere to that name, the more so as it is derived from a very ex- 

 tensive region of a great British colony. 



Having proved that the fossiliferous Lower Silurian zone is con- 

 formably surmounted by various crystalline flaglike strata, it follows 

 that the latter, though formerly looked upon as among the most ancient 

 rocks, must be simply viewed as other and younger masses of the 

 same natural Silurian group, but which have undergone such an 

 amount of metamorphism as to have obliterated the traces of any 

 animals which once inhabited the seas in which the strata were 

 accumulated. A glance at the little map* and table of colours, PL XII. 

 vol. xv., and a comparison of them with all preceding maps and 

 publications, will at once explain the changes which I have en- 

 deavoured to effect. The leading features of these changes I first 

 sketched out in the year 1854 at the Glasgow Meeting of the British 

 Association, and afterwards dwelt upon them at the meeting of that 

 body at Aberdeen in the autumn of 1859. 



Lastly, I would repeat the suggestion which I have before thrown 

 outf, that the stratified rocks of the north of Scotland are for the 

 most part equivalents in age of the Lower Silurian rocks of the 

 southernmost Scottish counties, the strata of which, having been 

 only partially altered, and having been left in the mineral condition of 

 " grauwacke," naturally exhibit much more frequently the evidences 

 of fossil organic remains than their highly crystalline and metamor- 

 phosed representatives in the northern Highlands. 



* Published in No. 62 of the Quart. Journ. Greol. Soc. 

 f Quart. Journ. Geol. Soc. vol. vii. p. 169 (1851). 



