and rent by volcanic forces, most probably when beneath the sea; and, at remitting intervals, molten lava was poured out of fissures generally in a line from north north-east to south south-west, partly over the sea-bed, and partly thrust in among the stratified rocks which it metamorphosed; and, cooling slowly, under the influence of considerable pressure, this lava assumed the stony crystalline character of basalt. Mr. Hopkins has shown how minor fissures are formed transverse to the chief line of fracture, and therefore it is highly probable that the transverse basaltic dikes are due to the same causes which produced the Whin Sill.
Some plants in Northumberland are confined to the igneous rocks, and chiefly to the augitic group. Dianthus deltoides is common to the porphyry and basalt: I have seen it on the very verge of the porphyry, but I could not, after diligent search, find a single specimen straying across the border to a stratified rock. Pyrola secunda, and Teesdalia nudicaulis, have not been noticed except on porphyry. But the following occur in Northumberland only on the basalt: Moenchia erecta, Sagina subulata, Vicia lathyroides, Asperugo procumbens, Statice limonium, Convallaria polygonatum, Allium schoenoprasum, Scilla verna, Sedum anglicum, Asplenium septentrionale, Asplenium germanicum. Helianthemum vulgare, although also occurring in limestone, I have sometimes found a useful guide in tracing the range of the Whin Sill.
POSTSCRIPT TO CHAPTER I.
ADDITIONAL OBSERVATIONS ON THE INFLUENCE OF THE SUBJACENT ROCKS ON PLANT DISTRIBUTION, BY J. G. BAKER.
From the more strictly botanical point of view I propose to supplement Mr. Tate's sketch by a few observations on the influence exercised by the character of the subjacent rocks on the distribution of the Flora. The sedimentary beds of the north-east of England fall under two well-contrasted types in their influence on the scenery and botany. These,[1] following the nomenclature
- ↑ Thurmann. Essai de phytostatique applique a la chaine du Jura. Berne, 1849.
