Vol. 51.] PROF. T. G. BONNET ON THE NARBOROUGH DISTRICT. 29 



giving bright colours with crossed nicols. Here and there occur 

 larger flakes of a pale olive-brown, somewhat dichroic mica, in 

 clusters, with an approach to a vein-like arrangement. There are 

 larger grains of opacite and ferrite, and streaks in which angular or 

 subangular transparent grains, about *001 inch in diameter or less, 

 are fairly abundant ; and occasionally slightly clear spots seem to 

 be forming. In one or two places the former presence of fragments 

 is suggested, but it is most difficult to explain the slight irregu- 

 larities of structure exhibited by the rock. Partly, no doubt, they 

 are due to original differences of composition, but partly they may 

 result from incipient crystallization. The rock on the whole presents 

 a certain resemblance to the slate at Brazil Wood, about 30 yards 

 from the granite. 1 The igneous rock, a small portion of which 

 (though it is rather rotten) has been retained in the slice, shows a 

 perfectly welded, rather wavy surface. In the groundmass, rather 

 ferrite-stained, perhaps once a glass, are felspar-crystals (replaced 

 by secondary products) from '02 inch in diameter downwards (the 

 shape being more suggestive of orthoclase) with small quartzes, 

 from about '002 to -003 inch in diameter, rounded in form, but ex- 

 hibiting sometimes one or two angles of 120°- AVith these are 

 grains of iron oxide and flakes of altered biotite. Pelspar-microliths 

 are not visible in the groundmass. 2 The other specimen exhibits 

 the same micas ; one, the more abundant, being commonly of a pale 

 olive-green colour ; the flakes generally are very small, but at one 

 part they are '006 to *007 inch in length, forming a vein-like mass. 

 The structure of the rock varies. One part probably contained 

 angular fragments of felspar, now replaced by secondary products ; 

 another, small fragments of quartz ; even a few fragments of vol- 

 canic glass possibly may have been present. As the general structure 

 of the selvage (except for the possible substitution of glass for felspar) 

 differs but Utile from that of the igneous rock of the pit, I conclude 

 that the mass at intrusion was at a fairly uniform temperature, and 

 this, judging from the effect on the sedimentary rock, was probably 

 not very high. 



As these rocks of the Narborough district present sometimes 

 resemblances, sometimes differences, both among themselves and in 

 regard to other igneous rocks in Leicestershire and Eastern War- 

 wickshire, I determined to ascertain what variations in specific 

 gravity they exhibited, and now append the results, obtained by 

 using a Walker's balance. 3 



1 Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 93. 



2 I have examined another specimen of the igneous rock of the quarry, but 

 need only add to my former description that the rock certainly has contained 

 biotite, and that the quartzes clearly belong to an early stage of consolidation, 

 for in some cases they are much corroded and occasionally are enclosed by a 

 thin ring of the groundmass. A fair idea of the microscopic aspect of this 

 rock may be obtained from the figure of the ' microgranulite de Luzy ' in 

 MM. Fouque and Levy's ' Mineralogie micrographique,' 1879 (pi. x. fig. 1). 



3 I am indebted to Miss C. A. Raisin, B.Sc, for much kind assistance. As 

 a rule the balance was independently read by both of us, and every pains taken 

 to ensure accuracy. When the results seemed in the least degree anomalous- 

 a second determination was made on another day. 



