part 4] PETROLOGY OF THE AKNAGE DISTRICT. 475 



xenolitlis and hornfelses are now described under the headings of 

 their original rock-types. 



Limestone. — Limestone forms part of the Arcllethen roof- 

 cake, but lias not been recognized as xenolithic in the contaminated 

 rocks. At. Arcllethen the hornfelsed rock consists mainly of large 

 grains of calcite, with a few small grains of pale monoclinic 

 pyroxene, tremolite-prisms, and scarce plates of pale mica. 



Fel spathic quartzite. — The hornfelsed quartzites of the 

 Fyvie Series are mainly coarse-grained massive rocks breaking 

 with a splintery fracture. They are composed of quartz in large 

 sutured grains often elongated in form, together with subordinate 

 smaller acid or medium plagioclase-grains similarly elongated. 

 There is occasionally a very pronounced parallel structure, as 

 if recrystallization had taken place under pressure. Sometimes 

 the felspar-plates are speckled with quartz-pellets, at others the 

 small felspar-grains are disposed around the edge of the large 

 quartz-grains. A few grains of zoisite have been noted in a 

 similar position. Biotite is of common occurrence, but usually 

 in no great amount. Garnet in rude pink grains is often noted. 

 In some rocks of this group the 'cement' between the quartz- 

 grains is formed wholly of cordierite. Certain of the quartzose 

 hornfelses of the Arcllethen district are extremely fine-grained, and 

 consist of an assemblage of minute grains of quartz, plagioclase, 

 and biotite. 



Hornfelsed andalusite-schists ; the slaty hornfelses. 

 — Certain of the argillaceous hornfelses from the country-rock 

 area of fig. 2 (p. 452) appear to represent the andalusite-schist of 

 the Fyvie Series, altered to a less degree than are xenoliths of 

 this rock found within the contaminated rocks. In this type 

 of hornfels the division between andalusite-porphyroblasts and 

 ground-mass of the original rock persists as a separation between 

 patches of cordierite and a more finely-grained base of quartz, 

 cordierite, biotite, and felspar. In the hand-specimen these rocks 

 are blotched with bluish patches of cordierite ; in slice, large areas 

 of cordierite, often 1 cm. across, are set in a fine recrystallized base 

 of biotite-laths, pleochroic from pale yellow to rich red, quartz- 

 grains, small cordierite-grains, and acid plagioclase-pellets. Often 

 the cordierite has been replaced by muscovite. But rarely does 

 andalusite persist in these hornfelses, and then it forms large 

 limpid crystals, with an intense patchy pleochroism from pink to 

 colourless ; this andalusite is quite free from inclusions, and in 

 this respect is in marked contrast to the inclusion-filled andalusite 

 of the original rock. A similar type of andalusite has been 

 described in a contact-metamorphosed andalusite-schist from 

 Cloichdubh Hill, Strathbogie. 1 



1 II. H. Read, ' Summary of Progress for 1918 ' Mem. Geol. Surv. 1919, 

 p. 31. 



