Paleo-Geological and Geographical Maps of the British Islands. 261 
Piate XXII. 
The Laurentian Period. 
This plate (Fig. 1), is intended to show those tracts where the Laurentian rocks 
reach the surface, and those under which they may be supposed to extend, though 
concealed beneath more recent formations ; also, the portions of the surface over 
the western part of the European area, including the British Isles, occupied by 
the land and sea of the Laurentian, or Archzean, period (Fig. 2). 
The Laurentian Continent (Atlantis)—As the Laurentian rocks form extensive 
tracts both in North America and Europe, it may be inferred that the land which 
was the source of the sediment of which they are composed was situated in a region 
lying between these two areas; that is to say, in the region of the Atlantic Ocean, 
including probably the continent of Greenland, and possibly the Polar regions. 
It may be supposed that large rivers flowed down into the ocean of the period, 
both towards the west and towards the east, and that this sediment was deposited 
over the floor of the Laurentian ocean, now occupied by North America and 
Europe. The margins of this land are necessarily only approximately inferential. 
Laurentian Areas (Fig. 1).—The Laurentian (or pre-Cambrian) rocks appear 
at the surface (over the area embraced by the map), as forming the greater portion 
of the Scandinavian promontory, in the north-western Highlands of Scotland, and 
outer Hebrides, in the north-west of Ireland and Galway, in the centre and north- 
west of France, and along the margin of the Silurian basin of Bohemia. They 
may be supposed to underlie all the remaining portions of the land, except those 
districts formed of intrusive granitic or trappean rocks, which, as compared with 
the former areas, are very small, and can only occasionally be represented on a 
map of the scale here adopted. 
Nature of the Laurentian Rocks.—These rocks consist of foliated granite or gneiss, 
hornblendic and micaceous schists, crystalline limestone or marble. The gneiss is 
generally massive, porphyritic, and of a red colour, consisting of orthoclase, 
oligoclase, quartz, and mica of two varieties. To this formation, the red granite 
of the Nile valley probably belongs. 
The Laurentian rocks have undergone intense metamorphism, owing to which 
they now only occur in a crystalline condition. Originally, there is every reason 
to believe, they were formed of sedimentary materials, such as those of the Lower 
Silurian system, consisting of sandstones or grits, slates, flagstones, and limestones, 
all of marine origin; and whether or not the Hozoon Canadense, found in this 
formation in Canada, be a true organism, the occurrence of beds of limestone leads 
us to infer that the ocean-waters of this early period of geological history were not 
destitute of living creatures, though probably of very simple organization. 
