INTRODUCTION OF IRRIGATION 661 



formation is more rapid than was that of the natural sediments observed, 

 in the shafts. At present our only way of estimating this rate is by 

 comparison with that of the accumulation of culture strata. Both the 

 city of Anau and the irrigation formation started on the natural surface 

 of the delta; and, while in the city the culture strata have grown to a 

 height of 38 feet, the irrigation formation has risen on either side to a 

 thickness of 15 feet, which would give a ratio of 1 of irrigation to about 

 2.5 of the culture strata of the city of Anau, which accumulated more 

 rapidly than those of the kurgans. 



Chronology 



The greatest interest centers, naturally, in the problem of the age of 

 these different cultures, and in their relation to the origin of Western 

 civilizations, if any relations may be shown to exist. 



The wide geographical separation between Anau and the fields of 

 Western cultures and the paucity of objects found by us that recall in a 

 definite manner similarities to objects of external civilizations surround 

 the subject with the greatest obstacles. 



Any treatment in the direction of proximate dating of any one of 

 the cultures of Anau or advancing a general chronological scheme can be 

 at the best only tentative and can serve only as a working hypothesis. 



Such a working hypothesis has gradually formed itself in my mind 

 and is developed in the following pages. 



To begin with, I assume — 



First, that distinctive pottery, peculiar to a culture throughout our 

 successively superimposed earth layers, is evidence of corresponding 

 continuity of that culture. 



Second, that since it is a fact that throughout the lives of our sites 

 at Anau the towns were built only of air-dried bricks, the secular rate of 

 growth of culture strata can be taken as proximatively uniform. 



Third, that two separate sites, whose cultures are characterized by en- 

 tirely different and peculiar potteries, can not exist contemporaneously 

 for centuries in close proximity to each other without such an inter- 

 change of pottery as would come to light during the excavation. 



This is applying to archeology the rules of geological reasoning. 



We know the thickness of the strata of each of the cultures of the 

 three neighboring sites and we know the aggregate existing thickness of 

 the cultures of each of the sites. 



If we take the duration of each culture to be proportionate to the 

 thickness of its accumulated strata, the duration of the entire series will 



