96 DE. A. H. COX: EEPOET ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
In addition to the stratified rocks enumerated above, the 
are known to occur :— 
following igneous rocks 
Upper Carboniferous or later. 
Pre-Carboniferous 
Pre-Cambrian . 
Olivine-dolerites . 
Granites. 
Granophyres of acid and 
basic types 
Andesites and dacites 
Intrusive in the Coal 
Measures, as dykes, 
sills, laccolites, or fault- 
intrusions. 
Intrusive into Charnian 
rocks (Mount Sorrel). 
Intrusive into Charnian 
rocks. 
Intrusive and ? as lavas 
in Charnian rocks. 
The greater part of the surface is occupied by various rock-groups of the Mesozoic 
Formations. They rest one upon the other in regular sheets, which dip, as a rule, 
gently eastward or south-eastward, so that newer rocks successively appear in that 
direction. Some of the sheets are, however, more or less lenticular in character and 
may wedge out and disappear altogether owing to overlap, thus complicating 
estimations as to the depth of the underlying formations. 
The Palseozoic and older Formations come to the surface from beneath the 
Mesozoic rocks only in the western part of the district. Though the Palseozoic 
(Carboniferous) rocks also dip eastwards as a general rule, between them and the 
Mesozoic Formations there is a strong unconformity, and the dips of the various 
Palseozoic rocks are not nearly so regular as, and are, as a rule, steeper than, those of 
the Mesozoic Formations above them. 
It will therefore be readily understood that the structure of the district, so far as 
the deep-seated rocks are concerned, is in part still unknown, and that it is not 
always possible to say definitely on what Formation the Mesozoic rocks rest. 
For these reasons it will be convenient to begin the description of the strata with 
the higher beds, the geographical distribution of which is definitely known. 
Both Palseozoic and Mesozoic rocks are affected by a system of east—west to south¬ 
east—north-west faulting. The distribution of this faulting, which has proved to 
have a definite relation to the distribution of the magnetic disturbances, will be 
discussed later (p. 111). 
2. Bocks above the Coal Measures. 
(a) Formations above the Marlstone Iron-ore. 
For our present purpose the Formations above the Marlstone (Middle Lias) Iron-ore 
may be dismissed with a brief reference. Apart from the Boulder Clay there is 
nowhere any cover to the iron-ore except at the extreme eastern margin of the 
district where the Marlstone is covered by the Upper Lias Clays, blue clays attaining 
