Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/385

372 of the several littoral sub-regions, as noted by myself at six British localities, three northern and three southern, each presenting a different condition of sea-bottom, and considerable variation in amount of rise and fall of tides. In the first column I give the distribution as observed at Sandwick, on the east coast of the mainland of Zetland, where the fall of the tide is very small, and the shore composed of steep rocks of gneiss. Extremely unfavourable as such a locality must be to animal and vegetable life, we see, nevertheless, that such species as do occur take their respective places with as much deference to right of prece- dence as in the more favoured localities with which it is compared. The second and third exhibited the littoral distribution as observed at Armadale, in the sound of Skye, at one point on rock (old red sand- stone), at another in a shingly bay. The presence of Lottia testudinaria in this column is a feature indicative of the Boreal type of the fauna. The fourth column exhibits the distribution as noted on lime- stone at Slade, on the sea-side of Hook-point, the southern extremity of the county of Wexford; and the fifth, on the old red sandstone coast in the same district. In both these we see evidences of a more southern fauna in the presence of Trochus crassus, a characteristic species of the South British type of our native mollusca, whilst the occurrence of Trochus umbilicatus here, as well as in the Armadale column, is characteristic of the western seas of Britain generally. In the Zetland and Armadale columns the Spirorbis marks a zone much more distinctly than in those of the southern localities. The sixth column is an incomplete (in consequence of the time of tide) note of the distribution on a Silurian rocky shore at Tramore, in the county of Waterford. In all these columns the several members of each sub-region are to be understood as intermingled, except when it is expressly stated to the contrary; and several of the members of the third and fourth sub-regions occasionally stray into the first and second. The abundance of animal life in the littoral zone and its respective sub-divisions must not be judged of from the number of species named in this table—my object at present being not to give lists of their flora and fauna, but to show how well marked, by characteristic forms, such zones are.

The importance of a knowledge of the characteristic features of the littoral zone, to the geologists investigating the northern drift, cannot be too strongly enforced, seeing that the evidences of them presented by fossils are the surest tests of ancient coast lines, and guides to the determination of the amount of elevation, or depression, and direction of action of the disturbing force.

The second, or Laminarian zone, is that land-encircling belt which commences at low water mark, and extends to a depth of from seven to fifteen fathoms. The great tangle seaweeds form miniature forests in