SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 657 
may be unsuspected by geologists, but may be detected through all the intervening 
strata by the magnet. Again, in Mid-Scotland and elsewhere, vast masses of basalt 
on the surface are associated with rocks newer than those which bound them both 
to the north and south. 
If the newer rocks hid the basalt a magnetic ridge line would appear to be 
associated with a synclinal axis. Anomalies are, therefore, to be expected, and we 
propose to bring together the cases in which our results, when interpreted on the 
magnetic rock theory, are in accord or in disagreement with surface indications, not so 
much with the intention of supporting the view to which, on the whole, we incline, as 
of collecting in a convenient form the arguments which may be adduced for or against it. 
The facts are clearly set forth in Map 14. The colouring of the map is explained 
in it. As the groups into which the strata are divided do not, except in the case of 
basic rocks, coincide with the main Geological divisions, the ordinary colours used in 
geological maps have not been adopted except for the basic rocks. 
To distinguish magnetic ridge lines from dykes the former are indicated by crosses. 
The magnetic indications appear to be quite independent of the disposition of the 
newer strata. 
Ihe Weald of Kent is a region of geological upheaval, but of low Vertical 
Magnetic Force. On the other hand, a magnetic ridge line runs across the geological 
depressions in North Yorkshire. 
Again : the well-marked ridge line which runs from near Heading to Chichester, 
has no apparent connection with the superficial geology. 
A mon^ the more ancient rocks the lower Carboniferous strata in Devonshire lie 
between the older Silurian and Devonian formations, but a ridge line traverses it 
right across the county. 
This is, however, almost the only exception, in England and Wales, to the rule 
that in the case of rocks older than the coal measures there is an approximate agree¬ 
ment between regions of geological upheaval and of high Magnetic Vertical Force. 
This is true of the Mend ip Hills. 
The line which runs through South Wales and Gloucestershire towards Reading, 
’ ranges from Pembrokeshire, where the older Palaeozoic rocks are mingled with trap, 
I through a region in which the Palaeozoic rocks are probably not far below the surface, 
i It may mark a line along which the older Palaeozoics are ranged, though covered 
! unconformably by newer Palaeozoic rocks. 
The line in Mid-Wales and Shropshire is for some miles parallel to the fault which 
runs from the Wrekin to Radnor. It starts where the Builth igneous rocks are exposed, 
runs by Wenlock Edge and the Wrekin—where again there is a mass of trap—and 
then passes direct from the older rocks in this neighbourhood towards those of Derby¬ 
shire. It certainly suggests an underground connection between them. 
Another ridge line connects the Nuneaton and Dudley Coal Fields which rise in the 
midst of newer rocks. The southward prolongation of this line towards Reading does 
MDCCCXCYI.—A. 4 p 
