102 DR. A. H. COX : REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
North Staffs. This rock carries nepheline, which has not so far been proved to occur 
in the intrusions among the English Coal Measures, so that it may belong to a quite 
different series of intrusions. Since no other basic rock is known to occur, in the 
Mesozoic rocks of the Midlands, we are warranted in assuming the probable absence 
of any basic intrusions in the Triassic and Jurassic rocks of the Melton Mowbray 
district. Accordingly we must look to the Coal Measures or older Formations for 
occurrences of these igneous rocks. 
(g) The Mo'imt Sor’vel (i-ranite and Associated Rocks. 
The Mount Sorrel Granite represents the only granite intrusion which is exposed 
within the district described in this report, or for a considerable distance in any 
direction outside. It is largely covered by an unaltered mantle of Triassic Rocks so 
that its limits beneath the mantle are not known, but it is obviously of considerable 
extent. The granite is intrusive into Charnian Rocks, but it is not affected by the 
north-west—^south-east (Charnian) movemenf which influences all the Charnian rocks. 
The granite is, therefore, presumably of much later date than the pre-Cambrian 
Charnian rocks. In no part of the country, however, is there any similar granite in 
Carboniferous or newer rocks, so that its age is almost certainly pre-Carboniferous. 
Petrologically it resembles most closely the “Newer’ or post-Silurian granites of 
Scotland.* If it should be of the same age as the “ Newer” granites of Scotland no 
stress could be laid on its apparently isolated nature, since any granite of corre¬ 
sponding age could not occur in Carboniferous or newer rocks. But such gi’anites 
are very common in the Highlands, and they appear in the Southern Uplands, in the 
Cheviots, and in the Lake district. Thence southward there is no opportunity of 
seeing any similar granite until Charnwood Forest is reached, but buried intrusions 
may well occur. Thus the Mount Sorrel intrusion may not in reality be as isolated 
as it appears, and there may possibly be other granites in the district buried beneath 
the Triassic or Carboniferous rocks. 
(h) The Charnian Igneous Rocks. 
Igneous rocks of several types are of frequent occurrence in the Charnian Rocks. 
The majority, if not all, are intrusive in character. Among those that are always 
defirutely intrusive are granopkyres of varying acidity. They occur abundantly, 
sometimes in large masses a mile or more across. Their exact limits are often 
difficult to determine on account of the overlying Triassic Rocks, but they have 
evidently a wide distribution as they are seen at intervals from the north of 
Leicestershire to the south before they finally disappear beneath the newer rocks. 
Porphyrotds, or altered andesitic rocks, are also common as large intrusions, and 
possibly in some cases as lavas. They attain a great development in the north¬ 
western part of Charnwood Forest, while similar rocks have been proved by a deep 
* W. W. Watts, “ Charnwood Forest,” ‘ Geology in the Field,’ vol. IL, 1910, p. 777. 
