VOLCANIC CRATER HAVENS 20I 



first stag-es of a sand spit's formation arc often advantageous 

 to the haven at the mouth of which they form. In the course 

 of time, however, as the drifting sands accumuhite, they force 

 the entrance to the port to the side toward which they are 

 travelling, and as they cannot readily pass the tideway chan- 

 nel provided the stream be vigorous, they accumulate on 

 either side of its mouth in broad shallows, such as are de- 

 scribed as lying in front of the passages which lead throuo-h 

 the lagoon barriers to the sea. 



In tideless seas or large fresh-water lakes these spits 

 are perhaps the most serious menaces to the harbors. ()win<'- 

 to the weakness or absence of currents to break through 

 the barriers which they form, they are apt to wall across the 

 inlet, leaving no larger channel than suffices to discharge 

 the waters of the rivers which enter the embayment. A 

 comparison of the spit harbors in our great lakes or other 

 non-tidal basins shows how important are these alternatino- 

 currents for the preservation, and often also for the con- 

 struction, of havens. Along shores of such seas and lakes 

 the spits are not so neatly formed as on the open sea-shore. 

 A large part of the migrating sand is sure to enter the 

 currentless harbor and shoal its waters. 



The group of volcanic crater havens will be mentioned 

 only in order to complete the list of the causes by which 

 harbors fit to shelter ships are formed. It is a familiar fact 

 that volcanic craters commonly have a cup-shaped form, and 

 in most instances the rim which surrounds the central cavity 

 is broken down at one or more places by the outrush of lava 

 or by the ruptures which are naturally formed during the 

 throes of an explosion. It is also a well-known fact that 

 by far the greater part of the well-preserved craters of the 



