Vol. 70.] INTRUSIONS NEAR MELROSE. 311 



The various dVkes of the district will now be dealt with briefly 

 under the respective headings trachyte, porphyry, felsite, and 

 quartz-porphyry. 



Trachyte. — Two purplish-pink trachytic dykes cross the main 

 road north-west of Earlston. The} r contain phenocrysts of albite, 

 surrounded b} r orthoclase, in a trachytic ground-mass. They 

 resemble the White Hill sill very closely. 



The Cauldshiels Hill dyke is a trachyte, composed of 

 sanidine-laths with beautiful trachytic structure, and an occasional 

 sanidine-phenocryst. Limonite replaces some of the felspar-laths, 

 and is also present in grains. There can have been very little, if 

 any, ferromagnesian mineral present originally. 



The riebeckite-trachytes, half a mile south-west of Cauldshiels 

 Hill, contain large kaolinized felspar-phenocrysts in a mottled, 

 fine-grained, purplish, trachytic ground-mass of sanidine-laths, 

 numerous tiny fresh riebeckite-prisms, and interstitial quartz. 

 Towards the centre of the largest dyke the structure approaches 

 the orthophyric. 



Porphyries. — On the shores of Cauldshiels Loch are 

 three porphyry-dykes which strongly resemble the trachytes just 

 described. They are greatly altered, and contain only kaolinized 

 felspars and limonite-pseudomorphs, possibly after riebeckite. 



Felsites. — A beautifully-banded felsite occurs as a dyke at the 

 top of Rhymers Grlen, close to the Birkhill Shales. The margin 

 which abuts against Silurian greywacke is fine-grained and sphe- 

 rulitic. Then follow alternate pale and dark- brown bands, the 

 latter opaque under the microscope. Towards the centre the 

 dyke is coarser, and presents an irregular fracture. Occasional 

 microphenocrysts of altered felspar occur in the fine-grained f elsitic 

 base, which, under crossed nicols, shows patchy devitrification. 

 Numerous quartz- veins traverse the dyke. 



Quartz -porphyries. — The dyke intruded in the Chief s wood 

 neck consists of small quartz and orthoclase- phenocrysts in a 

 compact pinkish-brown matrix. The quartz-phenocrysts give 

 rounded or square sections, and are subordinate to those of ortho- 

 clase. The latter are altered rectangular aggregates of muscovite, 

 calcite, and a little quartz. The ground-mass is a microcrystalline 

 aggregate of quartz, orthoclase, and some limonite. 



A long quartz-porphyry dyke, lying north-west of White law 

 Hill, contains large phenocrysts of deep-green pleochroic soda- 

 p} T roxene, kaolinized felspars, and idiomorphic quartz, in rectangular 

 and hexagonal sections. 



A 4-foot dyke, branching like a tuning-fork on the northern 

 shore of Cauldshiels Loch, carries phenocrysts of kaolinized 

 felspar and corroded quartz, the latter containing numerous in- 

 clusions of ground-mass. Irregular patches of limonite probably 

 represent some decomposed ferromagnesian mineral. 



