SILURIA. 
[Chap. II. 
granite, syenites, porphyry, &c., been altered at some spots into chloritic 
and micaceous schists, in others into quartz-rock, accompanied by most 
extraordinary flexures of the beds*. Agreeing with Professor Ramsay, 
Mr. Selwyn, and my other friends of the Geological Survey, that this is a 
true explanation of the case, a small sketch is here annexed, to indicate the 
amount of curvature which these metamorphosed strata have undergone, 
as seen at the promontory of the South Stack Lighthouse, near Holyhead. 
In Anglesea these contorted, crystalline rocks, with their intruded 
igneous rocks, are overlain by, and associated with, stripes or patches of 
different palaeozoic rocks, of Silurian, Devonian, and Carboniferous age, — 
thus forming a network which the most experienced geologist might have 
difficulty in unravelling. Their outlines are very accurately defined in the 
sheets of the Government Survey Map, and are well and fully described in 
the ' Geology of North "Wales > (1866) by Ramsay, pp. 164, 174, &c. The 
whole of the bands, with their southward extension for many miles along 
the edge of Carnarvon Bay, have a direction or strike from S.S.W. to 
N.BT.E., in common with that of the huge buttresses of the Cambrian grit, 
slate, and sandstone already noticed, which form the western flank of the 
mountainous range of Snowdon. (See the annexed Map.) 
It is to the lowest of these sedimentary rocks of the Silurian Region and 
North Wales, whether little changed as in the Longmynd, in a slaty con- 
dition in Carnarvonshire, or quite metamorphosed in Carnarvon Bay and 
Anglesea, that the term 1 Cambrian ' was restricted by the late Sir H. De 
la Beche and his associates the Geological Surveyors ; and I have naturally 
adhered to a definition which coincides precisely with my earliest concep- 
tion of the order of succession in the Silurian Region. 
The metamorphosis of the Cambrian rocks of North "Wales is an example 
on a smaller scale of the grand changes by which the Lower Silurian rocks 
of the Highlands of Scotland have been converted into gneiss, mica-schist, 
&c, as will be explained in the Eighth Chapter. 
* The reader will find an admirable account of with very great profit, was at that early date a 
some of the metamorphosed strata of Anglesea, truly remarkable proof of the geological powers 
and their contortions, in a memoir by the late of that eminent naturalist and most excellent man. 
Professor Henslow, published as far back as the Transact. Cambridge Phil. Soc. vol. i. p. 15. 
year 1822. This work, which may still be read 
