Beehy Thompson — Use of a Geological Datum. 221 



in Lower Lias times lower (greater combined thickness of Middle 

 and Lower Lias), because it, i.e. Orton, lay further to the north- 

 west of that line or direction about which, as a fulcrum, a general 

 north-westerly sinking was taking place at the time under con- 

 sideration, than either Gayton or Northampton. 



The almost identical thickness of combined Lower and Middle 

 Lias at Gayton and Northampton would lead us to judge that these 

 places were actually on the line of fulcrum, or one parallel to it, 

 and since they are almost accurately south-west and north-east 

 of each other respectively, that direction may be looked upon as 

 the general direction of the axis of movement/ 



Kingsthorpe. — The shaft at Kingsthorpe was made in a search 

 for coal, in 1836. According to the late Mr. S. Sharp, F.G.S.,^ "No- 

 accurate detailed section of the shaft was taken at the time ; but 

 at a depth of 210 feet from the surface, a water-yielding ' limestone 

 rock ' in the Middle Lias (Marlstone) was pierced, which produced 

 36,000 gallons of water per hour. At a depth of 880 feet (as is 

 stated in pencil notes on a diagram in my possession, which notes 

 are said to have been made by Dr. William Smith, 'the Father 

 of English Geology ') the New Eed Sandstone was reached, and 

 a flow of brackish water of a like volume to the former occurred." 



It is quite certain that the above description of the Kingsthorpe 

 shaft is in error somewhere, for, taking the figures as they are 

 given, (a) of the 210 feet down to and through the rock-bed of the 

 Middle Lias, the Upper Lias would absorb 180 feet or more, leaving 

 less than 30 feet for all beds between it and the Great Oolite Lime- 

 stone, whereas in the near neighbourhood they are considerably 

 more than twice this thickness ; (6) the top of the Middle Lias 

 appears as 57 feet higher than at the Kettering Koad boring ; (c) the 

 Middle and Lower Lias have an aggregate thickness of 654 feet 

 compared with 567 on the Kettering Road, a difference of 87 feet in 

 just over a mile. 



Applying the Marlstone datum to Kingsthorpe, the results come 

 out as below. Take + 107 feet O.D. as the top of the Marlstone 

 rock-bed at Kingsthorpe, the same as at the Kettering Eoad boring 

 quite near, and we get 267 feet (374 — 107) as its depth ; give the 

 rock a thickness of 4 feet, and it is evident that it would be pierced 

 at 271 feet from the surface. In 1881 the Kingsthorpe shaft was 

 opened up by my advice, to see if it were yielding any water from 

 the Marlstone at 210 feet or thereabouts ; it was not, but salt water 

 filled the shaft to within 270 feet of the surface exactly. Putting 

 these two items together, the inference is very forcibly driven home 

 that someone has mistaken 270 for 210 in reading the records of the 

 shaft, a thing very likely to happen. 



Personally, I have no doubt that the error suggested above 

 occurred, because, after making the correction, the general accuracy 



1 See also " The Victoria History of the Counties of England: Northampton- 

 shire," Geology, vol. i, p. 8. 



2 "Note on a futile search for Coal near Northampton " : Geol. Mag., Vol. YIII,. 

 p. .505. 



