279 



impregnated with secondary silica so that the bulk analyses show abnormal 

 silica percentages. When the veins in the Cheviot rock are wide the 

 central portions are composed of colourless quartz or chalcedony, the 

 marginal portions of a deep red substance. Dr. PETERSEN found that the 

 specific gravity of the colourless portion was 2 '440 and that of the red 

 portion 2'071 ; the percentage of water in the former being 2'89 p. c. and in 

 the latter 7'07. 



The rock itself is composed of labradorite, ferriferous, enstatite, augite, 

 magnetite, hematite (eisenglimmer), felspar microlites and a glassy base. 

 (Figs. I and 2, Plate XXXVI). The labradorite crystals give a 

 marked porphyritic character to the rock. They' frequently measure 

 from 2 to 3 mm. across. Under the microscope they are often seen to 

 be completely honeycombed by ramifying inclusions of the ground-mass. 

 Their outlines are frequently interrupted by inlets and creeks, also filled 

 with the ground-mass. Twinning on the albite plan is very common and is 

 sometimes associated with twinning on the pericline and Carlsbad plans. 



The most abundant ferro-magnesian mineral is a ferriferous enstatite 

 of the type characteristic of the pyroxene andesites. The forms in the 

 prismatic zone are usually well defined. The terminal faces are fre- 

 quently imperfect. The pleochroism is well defined in moderately thick 

 sections. Monoclinic augite is invariably found in association with the 

 rhombic pyroxene. The angles in the prismatic zone are not so sharp 

 as in the latter mineral ; indeed the mineral occurs rather as crystalline 

 grains than definite crystals. The extinctions in the prismatic zone vary 

 from to 44. The mineral is pale green in colour and devoid of 

 pleochroism ; so that in this rock the two pyroxenes may be readily dis- 

 tinguished by the simple test of pleochroism. 



The ground-mass in which the above constituents lie embedded as 

 porphyritic or micro-porphyritic elements varies in different specimens. 

 One common type consists of small felspars, magnetite grains, hexagonal 

 plates of hematite (eisenglimmer) and a colourless or slightly brownish 

 glass often containing globulites and longulites. Another type may be 

 described as a felted aggregate of felspar microlites lying in a glassy base 

 (mikrolithenfilz). This type occurs in rocks which show the most perfect 

 fluxion structure. The chemical composition of this rock and of the 

 glassy base which it contains has been already given on page 43. 



A somewhat exceptional rock to which the term augite-andesite may 

 be applied occurs in the crags above Langlee Ford on the south side of the 

 Harthope valley. Macroscopically it consists of porphyritic felspars 

 measuring from 2 to 3 mm. across embedded in a very dark compact 

 matrix. The specific gravity of the rock is 274 and the SiQ, percentage 

 is slightly over 60. Under the microscope the porphyritic constituents are 

 seen to be plagioclase and augite. The felspar is frequently honeycombed 

 with irregular inclusions of the ground-mass ; but their most striking 

 peculiarity is a cloudy appearance which can be shown, by means of a 

 magnifying power of 1,000 diameters, to be due to minute inclusions similar 

 to those frequently seen in the felspars of plutonic rocks. They exactly 



