132 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Yol.Y. 
the forests grew and matured; water would have to cover the same area 
to deposit the succeeding 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 tilled 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. 
