AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 93 
Irthlingborough. The nearest deep boring iii fact is at Orton,* about 15 miles 
N.N.W. of Irthlingborongli. There the Lower Lias was about 620 feet thick, and its 
base was reached at —292 O.D., below which were 49 feet of Rhaetic and other beds 
down to —341 O.D. Then came an igneous rock which was pierced to a depth of 
74 feet before the boring was abandoned. This rock compared closely, according to 
Prof BoNNEY,t with the porphyroids or altered andesites (dacites) of High Sharpley, 
Charnwood Forest. The buried Charnian Ridge of Pre-Cambrian rocks therefore 
rises at Orton to within 341 feet of O.D. The ridge probably trends south-eastward, 
that is towards Irthlingborough, following the usual Charnian strike, N.W. to S.E., 
but whether its depth beneath the surface remains constant in that direction remains 
unknown, as the Palseozoic floor has not been reached 'for a considerable distance 
either east or south-east from Orton.| 
Below the level of —500 O.D., therefore, Charnian rocks may well occur, and with 
them may be associated igneous rocks that might produce disturbances. But all this 
is purely speculative. Actually the Charnian rocks may lie much below the level of 
— 500 O.D., and there may be no associated igneous rocks of a kind to give rise to 
disturbances. The igneous rocks in the Charnian of Leicestershire all have small 
susceptibilities, with the single exception of the Mount Sorrel granite. The important 
point is that we can reasonably predict that for at least 700 feet below the North¬ 
ampton Ironstone there exists no rocks capable of producing any marked magnetic 
disturbances. 
As to the physical possihility of the disturbances arising at depths greater than 
700 feet, Mr, Walkee states :—“ The evidence from the lines of force is not absolutely 
definite in view of possible errors of observation. But it is at least sufficient to show 
that such a depth of origin is unlikely. The numbers given by station E suggest 
that the origin could not be deeper than even 300 feet without complications appear¬ 
ing in the results given by stations A and B. Further, even if the disturbances do 
originate so low as, or lower than, 700 feet below the surface, the rock stratum 
causing the disturbances would have to be such that its shape and boundaries corre¬ 
sponded with those observed at the surface. Such a coincidence is extremely 
unlikely.”—G.W.W. 
IV. SUMMAEY. 
The evidence so far obtained is in favour of the disturbances originating in the 
iron-ore bed itself, and consequently their agreement with results to be expected 
from the surface geology Indicates a real and not a chance relation between the 
disturbances and the geological structure. 
If further work should tend to confirm this agreement between magnetic dis¬ 
turbances and geological structure, it might open out the possibility of mapping 
* H. J. Eunsen, “ Palaeozoic Rocks beneath Northampton,” ‘ Q. J. G. S.,’ vol. XL. (1884), p. 491. 
t Op. cit. 
f See A. Strahan, “Pres. Address,” ‘ Q. J. G. S.,’ vol. LXIX. (1913), pi. A, p. Ixxviii. 
