286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1930 



paragneisses. We can, however, trace these sediments deep down 

 in the Archaean without any difficulty. 



On close examination of a conglomerate in these series of strata 

 we find as a rule also f)ebbles derived from still older sediments. 

 As a natural sequence we ask ourselves : Is it possible to reach the 

 bottom of the sedimentary formations ? 



WHEN WERE THE OLDEST SEDIMENTS FORMED? 



The sedimentary rocks are mostly deposited in water, and we may 

 without hesitation say that sedimentary rocks have been formed 

 as long as water has existed in liquid state on earth. 



When we have got so far in our discussion of the problem this 

 question immediately presents itself: When did water appear in 

 liquid state on earth? We can give no direct answer to the ques- 

 tion, but we can make the reflection that, as soon as the temperature 

 of the atmosphere and of the earth's surface falls below the boiling 

 point of water, the atmospheric water begins to condense in the form 

 of rain or dew. When the first rains fall, a new period of earth's 

 history begins. We can infer the conditions of this period by study- 

 ing the sediments, but we can only comprehend the extent of the 

 change occurring at the transition to this period when the conditions 

 on the earth's surface preceding the condensation of the water are 

 clearly understood. 



THE CONDITION AT THE EARTH'S SURFACE DURING THE 

 PREAQUATIC PERIOD OF THE EARTH 



During the time preceding the first condensation of the atmos- 

 pheric water, during the period which we may call the preaquatic, 

 igneous rocks and to a large extent also gneisses and other metamor- 

 phic rocks were formed in the earth's crust. Sediments such as we 

 now find covering the greater part of the earth's surface did not 

 exist, but we can not say that clastic rocks were wholly absent. 

 Loose material was produced as well by the tectonic movements in 

 the earth's crust, at this early period no doubt very violent, as by 

 variations in temperature. The bulk of the loose material on the 

 surface of the earth, however, consisted undoubtedly of volcanic ash. 

 This was carried far by the wind. No water basins existed, in which 

 the ash could get lodged, no water solutions could deposit 

 a cement binding together the grains. Where juvenile gases did not 

 effect cementation, the material remained a loose mass. 



An ash-covercd earth, no traces of water, no traces of life. A hot 

 surface of earth under a hot atmosphere, rich in water vapor and no 



