Sentence:  I have given my reasons for believing that … ; and that blank intervals of vast duration, as far as fossils are concerned, occurred during the periods when the bed of the sea was either stationary or rising, and likewise when sediment was not thrown down quickly enough to <WSD>embed</WSD> and preserve organic remains. 
Meaning of the word marked with <WSD> tags:  To lay as in a bed; to lay in surrounding matter; to bed.