AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 1 03 
boring to occur under a cover of newer rocks as far away as Orton, Northants (see 
p. 93), 25 miles south-east of the Forest, so that they also appear to have a wide 
distribution. 
(i) Other Igneous Roeks Possibly Present. 
Besides the various rocks described above which are knoivn to occur in the district, 
there may be other types of igneous rocks concealed underground, and although such 
other rocks have nowhere been -proved within the limits of the district, yet their 
possible presence needs to be taken into account, and it is desirable to enquire as to 
the likelihood of their occurrence. 
The Possibility of Basalts as Lava-flows in the Carboniferous Limestone. —It has 
already been mentioned (p. 99) that there is a likelihood of the Carboniferous 
Limestone occurring in its normal position below the Millstone Grit. This being so, it 
is desirable to enquire into the possibility of its including basalt lavas. Such lavas 
are extensively developed at the base of the Lower Carboniferous in the Midland 
Valley of Scotland, and are of local occurrence in Northumberland, Derbyshire, and 
Somersetshire, but in no other parts of England. But in the outcrops of Carboniferous 
Limestone nearest to Melton Mowbray—the small patches at Breedon Cloud, &c., 
north of Charnwood Forest—there are no lavas present, although the strata are on 
the same horizon as those that in Derbyshire contain the basalt flows.* Accordingly, 
the chances are that, if the Carboniferous Limestone is developed under the Melton 
Mowbray area, basalt lavas are not present. 
Camptonites” of the type Intrusive into Cambrian Rocks. —The possibility of 
Cambrian Bocks occurring deep underground in parts of the district has already been 
mentioned (p. lOO). If such Cambrian rocks are present they probably contain sills 
of camptonite similar to those that are so abundant in the Cambrian strata of 
Nuneaton. The latter have been proved by boreholes! to extend over a considerable 
distance between Nuneaton and Charnwood Forest. No Cambrian Bocks have, 
however, been proved on the east side of Charnwood Forest. The age of the 
intrusions is not known, but it is significant that in this country they are never 
found in strata newer than the Cambrian. 
II. The Belative Magnetic Susceptibilities of the Bocks. 
1. The Sedimentary Bocks. 
(a) Non-Ferruginous and Slightly Ferruginous Rocks. 
Those rocks in which the iron-content is so small as to be negligible were found, 
whatever their nature, to have susceptibilities of a very low order of magnitude, 
as illustrated by the fine-grained limestones (cement-stones) of the Lower Lias and 
* L. M. Parsons, “ The Carboniferous Limestone bordering the Leicestershire Coalfield,” ‘ Proc. Geol. 
Soc.,’ No. 1004, March, 1917, p. 56. 
t “ The Geology of the Leicestershire .... Coalfield,” ‘ Mem. Geol, Survey,’ 1907. 
