CAMBKIAN GROUP. 
481 
CHAPTER XXVIL 
CAMBRIAN AND LAUEENTIAN GROUPS. 
Classification of the Cambrian Group, and its Equivalent in Bohemia.—Up¬ 
per Cambrian Rocks.—Tremadoc Slates and their Fossils.—Lingula Flags. 
—Lower Cambrian Rocks.—Menevian Beds.—Longmynd Group.—Har¬ 
lech Grits with large Trilobites.—Llanberis Slates.—Cambrian Rocks of 
Bohemia.—Primordial Zone of Barrande.—Metamorphosis of Trilobites. 
—Cambrian Rocks of Sweden and Norway.—Cambrian Rocks of the 
United States and Canada.—Potsdam Sandstone.—Huronian Series.— 
Laurentian Group, upper and lower.— Eozoon Canadense^ oldest known 
Fossil.—Fundamental Gneiss of Scotland. 
CAMBRIAN GROUP. 
The characters of the tipper and Lower Silurian rocks 
were established so fully, both on stratigraphical and palte- 
ontological data, by Sir Roderick Murchison after five years’ 
labor, in 1839, when his “Silurian System” was published, 
that these formations could from that period be recognized 
and identified in all other parts of Europe and in Xorth 
America, even in countries where most of the fossils differed 
specifically from those of the classical region in Britain, 
where they were first studied. 
While Sir R. I. Murchison was exploring in 1833, in Shrop¬ 
shire and the borders of Wales, the strata which in 1835 he 
first called Silurian, Professor Sedgwick was surveying the 
rocks of North Wales, which both these geologists consider¬ 
ed at that period as of older date, and for which in 18t:}6 
Sedgwick proposed the name of Cambrian. It was after¬ 
wards found that a large portion of the slaty rocks of North 
Wales, which had been considered as more ancient than the 
Llandeilo beds and Stiper-Stones before alluded to, were, in 
reality, not inferior in position to those Lower Silurisfti beds 
of Murchison, but merely extensive und.ulations of the same, 
bearing fossils identical in species, though these were gener¬ 
ally rarer and less perfectly preserved, owing to the changes 
which the rocks had undergone from metamorphic action. 
To such rocks the term “ Cambrian ” was no longer appli¬ 
cable, although it continued to be appropriate to strata in¬ 
ferior to the Stiper-Stones, and which were older than those 
of the Lower Silurian group as originally defined. It was 
not till 1846 that fossils were found in Wales in the Lingula 
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