280 Paleo-Geological and Geographical Maps of the British [slands. 
cephalopoda, nautili, ammonites, and cuttlefishes, as well as other inhabitants of 
the deep. Saurians abounded both in air, land and water, and during the Oolitic 
period, living forms both vertebrate and invertebrate were excessively prolific. 
Although deep sea conditions generally prevailed throughout the Jurassic period 
owing to subsidence preceding more rapidly than deposition of sediment, yet, 
occasionally shallow lagoons were formed, such as those represented by the Stonefield. 
and Collyweston beds ; and towards the close, those of the Portland and Purbeck 
beds. 
Nature of the Jurassic Strata—Considered generally, the Jurassic system 
consists of two great divisions. The lower (that of the Lias), being argillaceous ; 
the upper (that of the Oolite), calcareous. But this description requires modifica- 
tion, as the Liassic beds contain in some places calcareous or arenaceous strata, 
and the Oolite great beds of clay and sand.* The different divisions, as Dr. Wright 
has shown, are characterized by different species of ammonites, which range not 
only over the English area, but into the Jura mountains, Switzerland and Germany. 
The total thickness of the group may be taken at 3,000 feet, of which the Liassic 
beds roach about 1,000 feet. 
Distribution of Land and Water.—How far the strata of the Jurassic group 
originally extended, and to what extent the higher elevations of the British Isles 
were covered by the waters of the Jurassic sea is a problem not easy of solution. 
At the same time, we have several indications of the former extension both of the 
Liassic and Oolitic strata which go far to guide us towards some definite con- 
clusions on this question. In the first place, having regard to the great thickness of 
these strata along the northern and western margins, we infer that they originally 
extended far beyond their present limits, and that they covered all the comparatively 
low-lying tracts of England now occupied by the Triassic strata. The existence 
of Rheetic and Liassic strata in the north-east of Ireland, and in the Vale of the 
Eden, near Carlisle, prove the original continuity of the Liassic sea with that 
which flowed over the central plains. These beds are also found skirting the 
western coast and islands of Scotland,} and on the east, the shores of Dornoch 
Firth. _Wemay well suppose, therefore, that the Jurassic sea bathed the flanks ofthe 
[rish and Scottish Northern Highlands, as shown in the map (Plate XXXI. Fig. 2). 
The western limits of the tract running along the Western Highlands of Scotland, 
are in part defined by the mountains of Derry, Donegal, and the ridge of the outer 
Hebrides. The Atlantic area was probably distributed into ridges or islands, with 
intervening sea-lochs and basins, of which Rockal, Bus (or Busse), and several of 
the little islets or sunken rocks in that portion of the Atlantic between lat. 10° and 
*T do not consider it necessary to enter into details which may be found in any of the text-books of 
Geology. 
{ As shown by Murchison, Geike, and Bryce. Quart. Journ. Geol. Soc., vols. xiv. and xxix. 
