1894. VOLCANIC DYKES. 115 



upon and among the Zostera, it is as nothing to that finding shelter in 

 the half-loamy, half-sandy soil which the roots of the Zostera, in their 

 increasing growth and decay, largely produce and firmly mat together. 

 Very numerous are the worms. The tube-mouths of the shy Sahella 

 [Branchio]nma)vesicnlosa opene very where. Siphunculids protrude delicate 

 starry crowns in equally large numbers. Errant worms, too, are 

 plentiful, and great gaping holes here and there bespeak the residence 

 of one or other of the burrowing prawns. Molluscs are everywhere. 



Examining the fauna of a drainage gutter, we get a wholly 

 different array of life-forms and a hundredfold greater variety. If the 

 great channels traversing the littoral from high- to low-water marks 

 be valuable economically as highways for the wracking carts ; if the 

 canal gutters be useful to the fishers as saving weary hours of pulling 

 against wind and tide, it is the drainage gutters — the least important 

 in geological origin — that have the most zoological value as the 

 breeding and sheltering grounds of innumerable animals. With very 

 many creatures the presence of the last-mentioned natural channels 

 makes all the difference between plenty and scarcity — indeed, I am 

 convinced that many species, quite abundant on these coasts, would 

 be difficult to include in the local fauna were it not for the advantages 

 afforded by these gutters. The dykes thus marked are less eaten out 

 than the others I have noticed. The dyke matter often comes to the 

 surface at their upper ends, from whence a rapid fall of level leads to 

 the outlet on the reef margin. 



Obliterate the traces of life around and all would betoken the 

 presence of a mountain torrent. Boulders of every size and shape 

 strew the steep and rugged bed ; this huge angular one, with riven 

 side, might but just have fallen with thundering din, detached by 

 frost-wedge from a neighbouring precipice ; these of lesser size, with 

 smooth surfaces and rounded edges, bespeak a long period of tumbling 

 and rolling hither and thither. Here, too, as in a mountain stream, 

 are bend-corners where, out of reach of the influence of the swift 

 central current, accumulate great patches of gravel and shingle. In 

 these drainage gutters littoral life flourishes in its greatest luxuriance. 

 They form the true centres of the life of the shore, whence proceed 

 in regular and ordered sequence vast colourising swarms of free- 

 swimming larvae. Two causes contribute. One is that, as these 

 channels run parallel with the coast, their inhabitants are protected 

 from the direct attack of storms, an important direct consideration to 

 the more delicate organisms, and indirectly to the larger,which, in turn, 

 feed upon the lesser. The second cause, and the greater, is, that these 

 gutters, as soon as the tide recedes below their level, become swift- 

 flowing streams, charged with minute animals and food-particles that 

 have drained from the myriad weedy surfaces and chinks that one 

 by one are exposed. This drainage-water, too, is rich in life-giving 

 oxygen, absorbed from the air by the fihns of water as they drain from 

 the tide-forsaken rocks. 



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