346 THE USE OF A GEOLOGICAL DATUM. [Aug. I5OI, 



26. On the Use of a Geological Datum. By Beeby Thompson, Esq., 

 P.G.S., F.C.S. (Read Juue 19th, 1901.) 



[Abstract.] 



A PROPER interpretation of geological phenomena frequently 

 requires that allowance shall be made for differential earth- 

 movements that have taken place since the period under con- 

 sideration. Present differences of level in rocks of the same age 

 may be due to actual differences in depth of the sea-floor on which 

 they were deposited ; but they may also be the result of subsequent 

 differential earth-movements. The rock selected as a datum should 

 combine as far as possible the following characteristics : — It should 

 be thin, of considerable horizontal extension, having similarity in 

 physical characters and palaeontological contents over a large area, 

 and situated as near as possible, in vertical sequence, to the reference- 

 deposit. In Northamptonshire three formations meet these require- 

 ments — the Hhaetic Beds, the Marlstone Hock-bed, and the Corn- 

 brash. The Author applies the Marlstone Rock-bed as a datum to 

 the study of the five chief deep explorations in j^orthamptonshire, 

 with the following results: — While the old land -surface (below the 

 Trias) now varies in height by more than 250 feet, the variation in 

 thickness of the rocks between it and the Middle Lias only reaches 

 56^ feet ; and although the old land-surface is actually lowest where 

 the Rhsetic rocks have not been detected, when compared with the 

 position of the Marlstone it is found to be the highest. The 

 further application of the same method enables the Author to 

 recognize Rhsetic rocks at jSTorthampton, to correct the record of the 

 Kingsthorpe shaft, and to explain the presence of Ti'iassic saline 

 water in the Marlstone. A revised section of the Kingsthorpe shaft 

 is given. Another point proved is that a general levelling-up 

 process was going on just before the beginning of the Lower Liassie 

 Period, and another at the close of the Middle Liassie Period. 



Discussion. 



The Rev. J. F. Blake and Mr, W. Whitaker spoke. 



The Author, in reply to Mr. Blake, said that he certainly thought 

 that the upper portion of the Marlstone Rock-bed was formed 

 at approximately the same depth over a very large area extending 

 from north-east to south-west. In directions at right angles to this 

 he would expect it to vary in character considerably. He thought 

 that the process illustrated in the paper might, with advantage, be 

 applied to the Lias and Oolites in other localities. 



