218 Beehy Thompson — Use of a Geological Datum. 



Gayton ; Northampton, Bridge Street ; and Northampton, Kettering 

 Koad — the Middle Lias is described in Mr. Eunson's paper a» 

 "Middle Lias (rock -beds)," and the thicknesses are given as 

 20, 20, and 21 feet respectively ; all below, for some 550 feet, 

 coming under the title Lower Lias Clay. No doubt, actually, the 

 Middle Lias, that is to say, all beds between the ' Capricornus ' 

 zone below and the ' Serpentinus ' zone above, is close upon 

 100 feet thick, but no information is available to fix the lower 

 limit precisely in any of the borings. 



The Littoral Beds are of somewhat different ages in the different 

 places, but have been grouped with the Trias or the Ehsetic for 

 convenience. 



Firstly, on reference to the Summary of Borings in Northampton- 

 shire, it may be observed that whilst the Old Land Surface now 

 varies in height by more than 252 feet, the variation in thickness 

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

 Q>&^ feet plus so much more as would require adding on to the 

 252 feet also. 



Secondly, it will be noticed that the Old Land Surface is lowest 

 where the Ebaetic beds have not been detected, which appears 

 rather singular. Mr. Eunson observed this, and made the following 

 remark thereon (p. 494) : " . . . but the Kettering-road boring 

 shows a great depi-ession. This may partly account for the absence 

 of the White Lias and Ehastic, and the sandy appearance and 

 uneven bedding which the lower part of the Lias Clay presented 

 in this boring, and which was not noticed at either Orton or 

 Gayton." 



My inability to understand in what way a quite local depression 

 could lead to an entire absence of certain characteristic marine beds 

 developed to a thickness of 36 feet within about five miles (Gayton), 

 and to sandy conditions of the Lower Lias, led me to make the 

 following simple calculations. Assuming that the well-marked 

 Marlstone rock-bed was deposited under uniform conditions as to 

 depth within the area under consideration, and taking it as a datum,- 

 then, according to the figures quoted in the Table of Borings, 

 relatively to Northampton, Gayton has since been raised 167 feet 

 and Orton 219 feet. A correction made on this basis altogether 

 changes the appearance of things. The Old Land Surface at 

 Northampton, instead of being 110| feet lower than at Gayton and 

 186|^ feet lower than at Orton, becomes 56^ feet and 32|^ feet higher 

 than at these places respectively. The following table will 

 graphically show this. 



Of course, in an argument of the nature here presented, it is 

 impossible to say how much of the difference of level is due to 

 upward movement of one place, and how much to depression of 

 another with which it is compared ; therefore the selection of a 

 section for reference is arbitrary. Having selected the Kettering 

 Eoad boring, Northampton, we will make a few comparisons 

 between this and each of the others, together with some observa- 

 tions arising; out of them. 



