OP THE 3IALVEE2T HILLS. 



493 



fresh, while the majority show indications of twin-lamellation. 

 Some crystals of apatite are present, and there is a little quartz 

 containing fluid-enclosures ; but the latter mineral forms only a very 

 small proportion of the rock, just enough to entitle it to the name 

 of quartz-diorite. Taking the coarsely crystalline structure of the 

 rock into account, it should, perhaps, rather be termed a hornblende- 

 gabbro. It does not appear to be foliated, and is probably an 

 intrusive rock. PL XIX. fig. 4 shows portions of the large horn- 

 blende and felspar crystals as seen by ordinary transmitted light and 

 magnified 55 diameters. 



No. 5. North Hill, near the top, South side. — A rather coarsely 

 crystalline dark iron-grey rock, consisting apparently of hornblende 

 and felspar crystals, the former being, as a rule, the larger. The 

 specimen exerts a moderately strong attraction when brought near 

 the magnetic needle. 



Under the microscope it is seen to consist of hornblende, triclinic 

 felspars, magnetite, and apatite. The felspar, judging from its 

 extinction-angles, is in most cases labradorite. It is considerably 

 altered, as a rule, especially on the borders of the crystals where 

 they abut against the hornblende, into a microcrystalline-granular 

 material, probably felsitic. 



The magnetite occurs in octahedra and in irregularly-shaped 

 patches, the apatite in well-denned crystals. 



There is no sign of foliation, either in the specimen or in the 

 microscopic preparation. Quartz appears to be absent; and the 

 rock may be regarded as a good example of a diorite. 



Xo. 6. North Hill (top). — A rather fine-grained, pinkish, granitic- 

 looking rock, showing slight foliation. 



Under the microscope the section shows felspar, quartz, horn- 

 blende, magnetite, apatite, and kaolin. 



The felspars appear to be, in part at least, triclinic, but as a rule 

 they are much decomposed. A very little mica may be present. 

 The rock is essentially a quartz-syenite or hornblendic granite 

 (PI. XX. fig. 1). There is no evidence to show that it is a meta- 

 morphic rock. Prom its rather fine texture it may be regarded as 

 .an intrusive sheet or dyke of no great extent. 



Xo. 7. North Hill, just south of Iv<j Scar Rock (bench-mark). — -A 

 very fine-grained dark-grey rock, resembling the whin of the Xorth 

 of England in general appearance. 



Under the microscope the felspars - show the twin-lamellation 

 characteristic of triclinic felspars, and the greater number of the 

 sections give extinctions of between 5° and 6° and between 15° and 

 16°. It would appear therefore that the dominant, if not the only, 

 felspar is labradorite. The other constituents are hornblende, mag- 

 netite, and apatite. The apatite occurs in well-formed hexagonal 

 prisms *, the hornblende in crystals which exhibit no regular boun- 

 daries, while the magnetite, which is plentiful, occurs in octahedra 

 or in irregular patches. Some opaque white matter is also present : 



* A group of seven individuals occurring in this rock is shown in PL XXI. 

 fig. 9. 



Q. J. G. S. No. 171 2 l 



