220 E. HILL AND T. G. EONNEY ON THE 



far as can be ascertained they belong to the monoclinic or triclinic 

 system, and perhaps are only hornblende. The line of division 

 between the above dark spots and the granite is tolerably sharp, 

 and it seems probable that they are included fragments of some 

 other rock almost melted down, rather than segregation nodes in the 

 granite. 



(5) The later Intrusive Rocks. — The principal dyke in the Old Pit 

 at Groby is on the western side. It runs in a general W.W. and 

 S.E. direction, dipping at a high angle to the east. The thickness is 

 variable, often about two yards, but we were informed that at one 

 place this had been considerably exceeded. At our last visit in 

 1877 it appeared to be thinning away at the north-western end of 

 the pit, and exhibited very interesting junctions with the syenite, 

 which it has broken up, injecting itself into fissures and enveloping 

 fragments. We obtained a specimen with a strip of syenite about 

 two inches wide, included between two pieces of greenstone. In 

 the thicker part this appears as a finely crystalline greyish-green 

 rock of pretty uniform colour and texture, except that here and 

 there are a few light- coloured felspathic spots. In the thinner 

 parts, and near the junction, it becomes quite compact, a little paler 

 and greener in colour, showing many minute dark granulations and 

 a slightly streaky structure parallel to its junction surface. A few 

 feet to the right of this is a another narrow, somewhat irregular 

 dyke, rather rotten and broken up, of a rock resembling the finer 

 variety of the last, very probably a branch from it ; but, as it was 

 said to thin out on approaching the floor of the pit, this has not yet 

 been proved. We were informed that another dyke had once existed 

 which had crossed the floor of the pit, making an angle of about 60° 

 with the first named, but all trace of this has now been quarried 

 away. 



Under the microscope the coarser variety of the first dyke exhibits 

 an irregular network of longish felspar crystals, mostly plagioclase, but 

 very much decomposed, or more or less replaced by the usual pseudo- 

 morphs, the interstices being occupied by viridite, hornblende, and 

 possibly a little chlorite. Opaque grains, apparently of magnetite, 

 are thickly scattered about, with a few larger hornblende crystals, 

 generally much decomposed ; there is also a little quartz, calcite, and 

 possibly dolomite. The finer variety, close to the junction with the 

 syenite, shows a number of small crystals, pseudomorphs after 

 plagioclase felspar, lying with their longer axes roughly parallel to the 

 junction surface, in a ground-mass tinged with viridite and thickly 

 crowded with black or nearly opaque brown granules. With crossed 

 prisms the ground-mass exhibits an indistinct microcrystalline struc- 

 ture with traces of a glassy base. These specimens probably repre- 

 sent only different conditions of the same rock. Though now horn- 

 blendic, I have a strong suspicion that they were once augitic and 

 true basalt. The percentage of silica in the more compact variety 

 is, according to one observer 51*4, according to another 52*4 (Prof. 

 Livein.g), which is low for a true diorite, but not too high for a 

 basalt (see p. 223). 



