7926 



SYSTEMATIC 



39 



Coast — thus filling up the 300-foot gap to abovit the extent which the 

 relative rates of deposition in the south and the midlands would suggest 

 (above p. 31) — has, at any rate, a deal of probability to recommend it. 



The results of this investigation seem to be working out in a manner 

 analagous to that which nearly forty years ago I demonstrated to be 

 the case with the Lias-Oolite (Cotteswold etc. Sands ; Q.J.G.S., xlv, 

 1889, 440) — in the South of England clay conditions lasted later than 

 north of the Mendip axis, sand also commenced there later, and so did 

 the limestone. 



The same phenomena obtain, in a broad sense, in regard to 

 Kimmeridge-Portland deposits, and may be shown by the following 

 Table, with the introductory remark that the use of the terms Portland 

 Stone, Portland Sand and Kimmeridge Clay, as if they connoted sequent 

 rocks of the same date right across country, is to be deprecated. These 

 terms are now coming into popular use in connection with economic 

 questions of agriculture, quarrying, building, drainage and water-supply. 

 Then the usual local sequence — Stone, Sand, Claj% as 



Portland Stone 

 Portland Sands 

 Kimmeridge Clay 

 is quite easily remembered and appreciated. But if the statement is 

 made that thick beds of what have been called Portland Sands in Bucks- 

 Oxon for about a century are Kimmeridge Clay, because they happen 

 to lie below beds equivalent to the rotundum clays of Kimmeridge, 

 confusion is the result. It has now to be recognized that the old 

 stratigraphical terms are useless in a strict scientific sense, but are 

 satisfactory as lithic designations when used with geographical qualifica- 

 tions — if, for instance, it be known that the Portland Sands of the 

 Midlands are much earlier in date than the Portland Sands of the South 

 Coast. For it is lithic constitution that is impwrtant in economic questions. 



For strict scientific work the Age names are available. 



The following Table IX shows the strict scientific Age terms and 

 the variation in dates of lithic conditions according to locality. 



This table is only a broad view of the case, for there are temporary 

 local lithic anomalies ; but it gives, in a general way, the sequences 

 of lithic events in the chosen areas. 



