ACTION OF SEA-WEEDS 55 



little bearini^ on the large problems we have to consider. 

 We have already remarked the fact that the supply of these 

 mills in which pebbles are ground to mud comes in the main 

 from the neighboring cliffs. On the north Atlantic coast, 

 and generally in all glaciated districts, a large part of these 

 pebbles are from points where the sea is assailing the easily 

 worn deposits of bowlders which were so plentifully accumu- 

 lated in the ice-time. Besides these waste materials on the 

 land, there is a large amount of the same kind of rubbish on 

 the floor of the sea, and much of it finds its way to the shore 

 in the following described manner : All along these shores 

 sea-weeds abound ; from the level of mean tide down to a 

 few fathoms' depth the rock-weed thrives, and in deeper 

 water, even to near one hundred fathoms of sea, the great 

 laminaria. or "devil's apron," grows wherever it can find 

 secure foothold. Sometimes these plants attach themselves 

 by their root-like bases — which are not in fact roots, for they 

 serve only for support — to shells which lie prone or are fixed 

 upon the bottom. More commonly they adhere to a pebble 

 left on the sea-floor by the melting glacial sheet, or drifted 

 out in the "pan-ice" which in winter forms along the sea- 

 margins. 



All these sea-weeds have floats which hold them upright 

 in the water, and as they increase in size, they pull on their 

 bases with constantly augmenting force. As the waves roll 

 over them they increase the tugging action, until linali}-, in 

 some time of storm, the plant lifts the stone from its bed 

 and floats it in the water, buoyed up by the vesicles of air 

 contained in its fronds. The plant and the uptorn stone are 

 together borne in by the heave of the sea onto the shore. 

 Coming into the breakers, the weed is c^uickly beaten to 



