1314 JIESSKS. A. HARKEK AND J. E. MARK ON 



A common featAire, which is also seen in the metamoi'phosed 

 Lower Limestone, is the occurrence of little ovoid or irregular nests, 

 a quarter of an inch to an inch in diameter, of little greenish or 

 liglit hrown crystals, which radiate not from the centre but from 

 one end of the nest. These little aggregates, which consist of a 

 monoclinic pyroxene mineral presumably an augite rich in lime, 

 may perhaps represent original nodular patches more purely calca- 

 reous than the rest of the rock. The mineral seems to be identical 

 with one already mentioned as an occasional constituent of the 

 metamorphosed andesites. In the little nodules it is usually bordered 

 by a zone of small felspar crystals (see PI. XII. fig. 2). 



The rhyolite fragments in the metamorphosed Calcareous Breccia 

 appear, in some instances, to have been altered by silicification 

 prior to the metamorphism, but others preserve the cryptocrystal- 

 line, microspherulitic, and other structures proper to them. Some 

 are traversed by curving perlitic cracks, now occupied b}' minute 

 veins of quartz or sometimes pyroxene. This latter appearance is 

 beautifully shown in polarized light, and is clearly due to the cracks 

 having been filled by minute calcite-veins, subsequently meta- 

 morphosed into a lime-silicate [1043]. 



Besides these fragments, there are often little round areas of 

 clear quartz, sometimes consisting of an irregular mosaic. These 

 correspond to the rolled grains of clastic origin seen in the non- 

 metamorphosed rocks, but the quartz has apparently been recrys- 

 tallized in situ [909 &c.]. 



The matrix in which the fragments are embedded is seen under 

 the microscope to consist of a densely packed aggregate of various 

 crystalline silicates, in which a few are sometimes developed in 

 larger crystals, mostly of ver)^ imperfect outline and arranged in 

 tuft-like groupings. The higher beds of the limestone have the same 

 general character. 



One common mineral is a colourless, brilliantly polarizing amphi- 

 bole, which may be referred to tremolite. Cross-sections show the 

 prismatic hornblende-cleavage, and longitudinal sections give ex- 

 tinction-angles up to about 16°. The transverse parting seen in the 

 tremolite of some metamorphosed limestones, e. g. in Glen Tilt, 

 Perthshire [1174], is not found here. The mineral sometimes 

 occurs in vein-like streaks or fan-like tufts [909] ; at other .times it 

 makes up almost the whole of the rock in particular spots [1215], 

 see PI. XII. fig. 3. The abundance of tremolite and other mag- 

 nesian minerals in these altered rocks, sometimes to the exclusion 

 of simply lime-bearing silicates like wollastonite, suggests that 

 the limestone may have been partly dolomitized before its meta- 

 morphism. 



Two pyroxene minerals are recognizable in the rock, often asso- 

 ciated in the same slide, building little crystalline grains and minute 

 granules. One of these, with a scarcely perceptible yellow tinge in 

 the slices, has the augite-cleavage and gives in longitudinal sections 

 a maximum extinction-angle of about 40°. Its other properties are 

 those of the augites, and, being doubtless a variety rich in lime, it 



