HIDDEN PERILS OF THE DEEP 



827 



mouths, pushing out the cost line and fill- 

 ing in the bottom. The main mouths of 

 the Mississippi are advancing into the 

 Gulf, but at a comparative!}' slow rate. 

 A break from the main river at Cubits 

 Gap, just above the head of the passes, 

 however, has done an enormous amount 

 of land-making, filling in an area of about 

 50 square miles between 1852 and 1905. 

 (See page 833.) 



The mouth of the Columbia River is 

 an interesting example of remarkable 

 changes of channel and of the movement 

 of an island. Vancouver in 1792 found 

 but one broad channel and no island. 

 The chart of 1851 shows two channels 

 separated by Sand Island, the northern 

 channel being the deeper. In 1851 the 

 center of this island was 2> l A miles south- 

 east of Cape Disappointment, in 1870 it 

 was 2% miles southeast, and in 1905 it 

 was i}4 miles easterly. Since 1851 this 

 island has thus moved 2 miles northwest- 

 erly directly across the middle of the 

 river entrance, closing up the northern 

 channel and leaving the river with a sin- 

 gle channel, as in Vancouver's time. 

 (See page 834.) 



REMARKABLE CHANGES CAUSED BY 

 VOLCANOES AND EARTHQUAKES 



Volcanic action in well authenticated 

 cases has caused islands to rise or disap- 

 pear. In the present location of Bogoslof 

 Island, in Bering Sea, the early voyagers 

 described a "sail rock." In this position 

 in 1796 there arose a high island. In 

 1883 another island appeared near it. In 



1906 a high cone arose between the two, 

 and a continuous island was formed 

 about 2 miles long and 500 feet high. In 



1907 this central peak disappeared, and 

 in its place there is at present a bay with 

 from 4 to 25 fathoms of water. Bogos- 

 lof is an active volcano, and the main 

 changes have been the result of volcanic 

 action. The history of this island for 

 over a century past forms a remarkable 

 record of violent transformations. 



Earthquakes sometimes cause sudden 

 displacements, horizontal or vertical, of 

 sufficient amount to affect the informa- 

 tion shown on the charts. A careful in- 

 vestigation of the effects of the earth- 



quake in Yakutat Bay, Alaska, in Sep- 

 tember, 1899, showed that the shore was 

 raised in some parts with a maximum 

 uplift of 47 feet and depressed in other 

 parts, and that at least two reefs and 

 four islets were raised in the water area 

 where none appeared before. Un- 

 doubtedly there were changes in the 

 water depths, but definite information is 

 lacking because there had been no previ- 

 ous hydrographic survey. 



The San Francisco earthquake of 1906 

 caused little vertical displacement, but 

 there were horizontal changes of relative 

 position as much as 16 feet; so far as 

 known, this earthquake did not affect the 

 practical accuracy of the charts. The 

 Italian government recently announced 

 that the terrible Messina earthquake of 

 1908 has had no effect on the navigability 

 of the Straits of Messina. 



Related to earthquake phenomena are 

 the gradual coast movements of elevation 

 or subsidence which are taking place, but 

 at so slow a rate as not to sensibly affect 

 the charts in ordinary intervals of time. 



Another agency at work is the coral 

 polyp on the coral reefs ; although the 

 rate of growth appears to be very slow, 

 the resulting reefs and keys are an im- 

 portant feature in tropical seas. 



Changes in apparent shore line as 

 shown on the charts are also caused by 

 movements in the fronts of glaciers dis- 

 charging into the sea. An instance of 

 this is the recession of Muir Glacier in 

 Alaska. As a result Muir Inlet in 1907 

 extends 7% miles farther north than in 

 1895, an d a mountain of 1,020 feet ele- 

 vation on its eastern shore has been un- 

 covered by the departure of the ice. 



Practically all the land features shown 

 on charts are likewise subject to changes, 

 the more rapid of which are mainly due 

 to the works of man. 



The changes of channels and of com- 

 mercial needs cause many alterations to 

 be made from time to time in the lights 

 and buoys which are shown on the charts. 



KEEPING OUR CHARTS UP TO DATE 



The problem of keeping a chart suf- 

 ficiently up to date is one of much prac- 

 tical importance and one which must be 



