22 
SILUKIA. 
[Chap. II. 
North America, strata formed during the earlier periods are, throughout 
extensive regions, composed of ordinary sandstone, shale, and limestone ; 
but when pursued eastwards across the undulations of the Appalachian 
chain to the granitic rocks of the sea-board, or westwards to the Bocky 
Mountains, these sediments are found to have been converted into masses 
more or less crystalline. 
This is the change which geologists call metamorphism, and which was 
alluded to in the First Chapter. 
It will presently be shown that the comparatively low and unaltered hills 
of Shropshire and Herefordshire, and some adjacent tracts of Wales, present 
to the student, in an intelligible manner, a succession of strata, which has 
been with much difficulty deciphered amidst the hard, rugged, metamorphic, 
and crystalline formations of the same age in North Wales — even when 
most of the obscurities had been removed by the labours of the eminent 
persons who, from Professor Sedgwick downwards, have toiled in the ardu- 
ous task of bringing the geology of that region into order. 
In Russia, the Silurian rocks form either wide level plains or low 
plateaux ; whilst in other countries, where they have been heaved up into 
mountains, they have a rounded outline, especially where they consist of 
schists, originally composed of mud, the fine grains of which have given 
rise to equable atmospheric attrition. When, on the contrary, the shale and 
schist have been changed into hard slates, the sandstone into quartz -rock, 
or the earthy limestone into crystalline marble, and particularly if the beds 
be highly inclined and penetrated by igneous rocks, then sharp peaks or 
abrupt cliffs and gorges are dominant. Thus it is that the same ancient 
strata of different regions put on so many different external forms. In 
South Britain they are, necessarily, most varied in districts which, like 
those of North Wales and Cumberland, have had their outlines diversified 
by the intrusion of igneous rocks. 
Observation has now taught us of what materials the fundamental rocks 
of different countries consist. In Scandinavia, particularly in the central 
and northern parts of Norway, there is every reason to believe that, as in 
British North America, Bohemia, and the North-west of Scotland, crystal- 
line rocks of Laurentian age underlie all the deposits to which the terms 
Cambrian and Silurian can be applied. In Bohemia, however, as in Great 
Britain and portions of North America*, the lowest zone containing Silu- 
rian remains (' Zone primordiale' of Barrande) is underlain by very thick 
basements of earlier . sedimentary accumulations of Cambrian age, whether 
* In North America the Huronian rocks are, 
as far as known, azoic. Banging along the northern 
borders of Lakes Huron and Superior, they dis- 
appear in Wisconsin and Minnesota beneath the 
next succeeding formation, the Potsdam Sand- 
stone, or lowest set of strata in which Silurian 
fossils are known. This 'Huronian System' of 
Logan is mapped in detail in the Atlas of the 
Geol. Beport of Canada, 1863-5, where it is 
shown that the Lower Silurian rocks are sepa- 
rated from the Laurentian by eleven zones of 
these Huronian schists, quartzites, conglomerates, 
chert, and limestone, thus presenting a full equi- 
valent of the Cambrian, strata of Enrope. See 
also Dr. Bigsby's elaborate comparison of the 
Cambrian and Huronian rocks, Quart. Journ. 
Geol. Soc. vol. xix. p. 36 &c. 
