III. 



The Influence of Volcanic Dykes upon Littoral 



Life and Scenery. 



ALONG the coast of Jersey there stretch for miles great masses of 

 rocky, weed-grown reefs, laid bare with every tide ; reefs cut 

 and hewn into every imaginable form ; channelled, and divided and 

 sub-divided by numerous broad, artificial-looking roadways, and by 

 interminable, intricate, mazy runnels. 



If we examine these, calling to our assistance pick, and shovel, 

 and geological hammer, we find all these paths and bye-paths to be 

 the outward and visible signs of the presence of volcanic dykes that 

 everywhere, in this fire-visited island, intersect and pierce the mother 

 rock. The latter may be diorite, syenite, or schist ; all are treated 

 alike, and the resultant features closely approximate. As a rule, the 

 dyke material is diabase, but minette (micaceous porphyry) is not 

 uncommon. This filling, sometimes softer, and always more joint- 

 riven, is much more rapidly disintegrated than the surrounding rock. 

 Exposed to a violent tide — rising sometimes fully 42 feet — and to a 

 sea occasional!}^ very stormy, the disintegrated matter is quickly 

 removed, and a ditch, often extremely well marked, is formed. This 

 may vary in breadth from a few inches up to twenty yards, or even 

 more. 



The faunal features of these wave-worn dykes vary considerably, 

 consequent upon whether their direction is at right angles to, or is 

 parallel with, the coast. The former are poor in life, barren, and 

 often sand-choked, but of great value to the agricultural community 

 as cart tracks in the wracking season — that harvest time on the shore, 

 when every farmyard in the island sends out all hands and carts to 

 levy toll from the bleak rock-reefs of their abundant weedy crop. 

 Many of the more marked of the eaten-out dykes serve this purpose, 

 and it is difficult indeed for those who know only shores with a 

 narrow littoral, to appreciate the vast importance of these natural 

 cartways on a coast such as that of Jersey, where a distance of from 

 one-and-a-half to two miles frequently separates high-water mark 

 from the Laminarian zone, whose weedy growth is more greatly prized 

 by farmers than the higher growing and commoner fucus. 



Besides thus serving as ready-made roads to the far-distant lower 



