242 ANNUAL OF SCIENTIFIC DISCOVERY. 



the breaking up of ice in the spring, but on a much grander scale, the 

 stream of lava being five or ten miles in width, and in place of smooth 

 ice., shaggy heaps of black scoria many yards in thickness. These 

 clinker districts are often several miles in breadth, and upon some of 

 them the whole horizon around is one wide waste of gray and black 

 desolation beyond the power of words to describe. In the winter 

 Mount Kea is covered with snow, while Loa, owing probably to the 

 internal fire, is almost bare. N. Y. Literary World. 



A NEW THEORY OF THE EFFECT OF THE TIDES. 



AMONG the discoveries in science recently made on this side of the 

 ocean is one which has excited much attention and interest among 

 geologists and navigators ; we mean the tide-theory of Lieutenant Davis, 

 U. S. N., first presented at the meeting of the American Association 

 in September, 1848. The following sketch of the principal results at 

 which Lieutenant Davis has arrived was prepared by Mr. Desor, for 

 Sillimari's Journal. 



The eastern coast of the United States is bordered throughout its 

 whole extent by a line of sand-banks and islands of various forms and 

 outlines, but very uniform in their mineralogical character, being com- 

 posed of a very fine white and quartzose sand. On the coasts of the 

 Southern States, they form a line of low islands separated from the 

 coast by a series of lagoons, which give a peculiar character to the 

 navigation of those districts. 



Higher up, on the southern coasts of New England, they occur as 

 submarine ridges, parallel to the coast, and separated from each other 

 by wide channels. Farther north, these deposits are more extensive, 

 and form vast submarine plateaus, such as the St. George's and New- 

 foundland Banks. Finally, deposits analogous to these are formed at 

 the bottom of bays, but in a state of more complete trituration. 

 These are known under the name of flats. 



Mr. Davis, after having devoted several years to the study of these 

 various species of banks, has arrived at this result : that their forms, 

 extent, and distribution are principally determined by tides ; the wind 

 and the waves playing but a subordinate part in their formation. One 

 of the first points on which Mr. Davis insists is the relation that ex- 

 ists between the strength of tides and the distribution of sand-banks. 

 On both sides of the Atlantic we invariably find sand-banks most nu- 

 merous where the tides are slight, or where their force is exhausted after 

 having been considerable. Mr. Davis accounts for this in the follow- 

 ing manner : According to the researches of Mr. Whewell, the 

 tidal wave, on entering the Atlantic Ocean, passes onward in the form 

 of an arc ; the convexity of which is turned toward the north. In its 

 progress northward, this wave strikes against the coasts of the two 

 continents of Africa and America. From this shock proceed the vari- 

 ous local currents which are designated under the name of tidal cur- 

 rents, the direction and rapidity of which are determined by the shape 

 of the coasts. Their rapidity is, in general, in proportion to the di- 

 rectness of the obstacles opposing them, and the narrowness of the 



