ARTICLES 85 



yet been traced to any definite horizon ; and the majority of 

 the specimens from these altitudes are waterworn, and are 

 therefore usually regarded as derived from the 100 ft. terrace. 

 Here, again, further observations are desirable, and any evi- 

 dence pointing to Chellean implements being contemporar}^ 

 with low-lying gravels should be carefully noted. 



The recurrence of a pre-Chellean industry at two different 

 levels, and of a Chellean at three, coupled with the frequently 

 close parallelism between the geological deposits of the different 

 terraces, offers a problem which can be solved in two different 

 ways : 



(1) The Similarity of Industries and Deposits on any two or 

 more Terraces indicates Contemporaneity. — In this case each of 

 the three deposits (two gravels and an intermediate shelly marl) 

 on the second and third terraces at Menchecourt and Abbeville 

 must in turn have reached a thickness of more than 30 ft., 

 and then have been almost entirely removed before the next 

 one was laid down. That is by no means impossible, but the 

 coincidence of the three deposits behaving in exactly the same 

 way at long intervals of time must be fairly faced. 



(2) The Similarity of Deposits indicates the Recurrence of 

 a Geological Cycle, probably under similar climatic conditions, 

 but separated by an interval of time during which the river 

 changed the level of its bed by about 30 ft. On this view, 

 when the pre-Chellean industry left the third terrace (assum- 

 ing that to be the earlier), it did not become extinct, but merely 

 migrated, to be succeeded after an interval by the Chellean 

 culture ; and when suitable climatic conditions recurred, each 

 industry in turn came back to the same latitude as before, 

 but owing to the change in river-level, occupied a lower 

 position. 



The first hypothesis seems to be the one generally adopted, 

 but in the absence of any conclusive evidence, it is well to bear 

 the second also in mind as at least a possibility ; and some 

 further light is thrown on it by a comparison of the first and 

 second terraces at Montieres and St. Acheul respectively. In 

 each case we find two distinct layers of gravel, overlaid by a 

 clay or marl called by the workmen terre-d-pipe ; but the simi- 

 larity of deposits does not indicate contemporaneity ; for the 

 lowest gravel is pre-Chellean in one case and typical Chellean 

 in the other ; while the second gravel and the terre-d-pipe are 



