132 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Voir. 



the forests grew and matured; water would have to cover the same area 

 to deposit the succeedmg stratum; and again this stratum would have 

 to rise above the water before a second forest could grow. There are 

 two ways in which this result could be brought about. In a district 

 subject to such intense volcanic action as this must have been, a succes- 

 sion of minor oscillations might have been associated with the general 

 subsidence, so that large areas of the lake border districts would be 

 alternately above and beneath the sea, or, as was doubtless often the 

 case, the shallow portions of the sea became filled up with the rapidly 

 accumulating ejecta, and sub-aerial deposits of sufficient depth were laid 

 down to allow the growth of forests, which, in time, were depressed 

 by the general subsidence, to be buried by a succeeding stratum of the 

 volcanic debris. But this latter method was not the ordinary one, as is 

 attested by the fact that many of the forests have grown in beds of fine- 

 grained material that must have been formed beneath the surface of the 

 water. 



I shall, however, not attempt to pursue this matter farther until all 

 the data and materials collected have been examined. A thorough study 

 of the various volcanic rocks will probably throw much light upon this 

 very interesting group of strata. 



I 



