28 H. JONSSON 



during neap-tide 7 feet, and it is also recorded from the latter place 

 that the low-water at neap-tide lies about 3 feet above that at 

 spring-tide. 



When extraordinary conditions prevail the height of the flood- 

 tide may be still greater; thus, 18 19 feet has been recorded from 

 Reykjavik and 18 feet from Stykkisholmur. 



From this it is seen that the height of the flood-tide varies 

 greatly, which must necessarily affect the algal vegetation in several 

 respects, especially as concerns the upper limit of its growth. Thus 

 there is a great difference in the highest flood-mark (at spring-tide) 

 and in the lowest flood-mark (at neap-tide). The tidal wave or the 

 tide, moreover, shows irregularities, because neither the highest 

 flood-mark (spring) nor the lowest flood-mark (neap-tide) is constant. 

 The limit of the flood-tide fluctuates perpetually between a high- 

 mark and a low-mark. The high-mark or the highest limit of the 

 flood-tide is seen distinctly on rocky coasts from the action of the 

 sea on the rock. On flat coasts, the high-mark can be distinguished 

 by detached algae and various other bodies which accompany the 

 tidal wave and remain at the highest level reached by the water. 

 This high-mark lies considerably higher than the upper limit of the 

 algal vegetation. 



The low-mark is not as easy to distinguish as the high-mark, 

 but it will almost coincide with the upper limit of the Pelvetia- 

 Fucus spiralis association. Above this the Bangia association (Ulo- 

 thrix, Bangia, and others) is found, the extreme limit of which will 

 almost coincide with an average water-level which, however, does 

 not lie midway between low-mark and high-mark, but rather nearer 

 the former. The upper limit of the algal vegetation thus lies some- 

 what above the limit of the flood-tide at neap-tide. The same rule 

 holds good, of course, on a very exposed coast. That the place is 

 exposed means that there is a heavy swell, which causes the sea 

 to rise higher up on the coasts, both at neap-tide and at spring-tide. 



By the upper limit of the marine algal vegetation is meant that 

 boundary line above which marine algae do not occur in the form 

 of associations; on the other hand, no account is taken of single 

 individuals or groups of individuals being found higher up, in 

 crevices or pools, as such an occurrence must be considered accidental, 

 because they are carried up to this height with the high water or 

 by far-reaching breakers. They do live, of course, but do not thrive, 

 and have evidently gone beyond their real area of distribution. In 



