AND LEICESTERSHIRE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. J 31 
The rock from Kelham boring (p. 101) is in some respects more similar to the 
Whitwick rock. It is an ophitic to subophitic teschenite containing angite and 
serpentinous pseudomorphs after olivine. It is rich in analcite and the felspars are 
partly analcitised. It difiPers from the Whitwick rock in being somewhat fine¬ 
grained, and in containing an abundance of some dark-brown interstitial material 
which is probably a glass free from crystallites of iron-ore.* 
Iron-ore constitutes about 4 per cent, of the rock, and is partly in the form of rods 
and partly as square, rectangular, or rather irregular grains up to O’l mm. in diameter. 
It is evidently proportionally richer in magnetite than the typical Southwell specimen. 
The susceptibility was 279 x 10“® after demagnetisation. 
2. The Mount Sorrel Granite and Associated Rocks. 
There are two main varieties of the granite—a grey and a pink variety. In the 
Mount Sorrel quarries these two varieties occur intricately intermingled one with 
the other, but apart from the colour, there does not appear to be any marked 
difference. 
With the normal granite there are associated various types of more basic rocks. 
These include (i.) the basic marginal granite, (ii.) the basic patches or “heathen,” 
(iii.) the diorite of Brazil Wood, and (iv.) basic dykes. 
The grey variety is a typical coarse-grained, non-porphyritic granite composed 
of plagioclase and orthoclase felspars, hornblende, biotite, quartz, and accessory 
minerals, the latter including magnetite and sphene. 
The plagioclase is present in somewhat greater amount than the orthoclase and 
it builds larger crystals, which are often well formed and are strongly zonal, the 
composition of the central portions being that of a medium andesine varying to an 
acid oligoclase in the outer portions. The mineral is generally rather fresh, but 
it may show the usual alteration which is sometimes guided by the zoning. The 
orthoclase, especially in larger crystals, shows a patchy extinction owing to perthitic 
structure. The relations between orthoclase and quartz are variable, sometimes the 
one, sometimes the other showing the better crystal form, but only occasionally are 
there any indications of graphic structure. The hornblende is pale green in colour 
and builds ragged crystals which occasionally enter into a sort of graphic intergrowth 
with quartz. It is usually subordinate in amount to the biotite, which latter mineral 
shows the characters usual in granites. Calcite occurs in small amounts as an 
interstitial constituent or replacing hornblende. Sphene occurs as large wedge-shaped 
crystals and as irregular granules, many of the latter being included in biotite. 
Pyrites is present locally as irregular masses in felspar spreading in dendritic fashion 
partly guided by the felspar cleavage. Magnetite occurs as grains enclosed in any 
* J. S. Flett in “ The Concealed Coalfield of Yorkshire and Nottinghamshire,” ‘ Mem. Geol. Survey,’ 
1913, p. 58. 
