288 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 



How, then, were these oldest subaqiiatic sediments constituted 

 and how did they occur? 



THE OLDEST SUBAQUATIC SEDIMENTS 



The sediments deposited in connection with the first appearance 

 of the seas were of a peculiar type. The material was entirely 

 eruptive, mainly volcanic ashes. A sorting of it according to the 

 size of the grains took place in connection with the transportation 

 but, the material being for the most part and on large areas very 

 fine and uniformly grained, this should have led to no more pro- 

 nounced stratification. 



As the material was loose from the beginning, it could immediately 

 be redeposited without previously being loosened b}^ weathering. 

 Hence it follows that the oldest sediments to a large extent must 

 have possessed the same mineralogical and chemical character as 

 the igneous material (or, to put it more exactly, as the volcanic 

 ashes) from which they were formed. Thus it was not clays, lime- 

 stones and sandstones that constituted the first subaquatic series of 

 strata but pyroclastic rocks more uniform in their character. 



The pyroclastic sediments grew rapidly thicker in larger as well 

 as in smaller basins. In most places the subaquatic sediments rest 

 on aeolian material, and it is not possible to show any petrographical 

 difference between these aeolian and subaquatic parts of the series 

 of strata. 



Upon the first, short period, with the formation of exclusively 

 pyroclastic sediments, a second one follows during which the chem- 

 ical effects of the water begin to be noticeable in the sediments. The 

 leaching out of alkali, lime, and iron was, as mentioned, greater than 

 during any other period in the earth's history, and probably the de- 

 position of lime and iron compounds was very abundant, even though 

 locally limited. A precipitation of for example ferric hydroxide 

 might then as now take place on a very different scale in different 

 basins. Differences in accessions, in the circulation of water, in the 

 evaporative conditions, etc., interrupt the uniformity otherwise char- 

 acterizing these early periods. 



We have hardly occasion in this connection to follow the develop- 

 ment further. The next step in the subaquatic sedimentation fol- 

 lowing immediately upon the one just mentioned, is the formation 

 of sediments abundant in clay and quartz as well as those rich in 

 lime and iron. We then have such a normal formation of sediments 

 as may be followed through the geological periods up to our time. 



If the first subaquatic sediments have possessed a peculiar charac- 

 ter and have had a magnificent development, one must ask if some- 

 thing of them is not to be found in the earth's crust. 



