186 KYNASTOX : VOLCANIC ROCKS OF THE CHEVIOT HILLS. 
various points along the margin, especially in the river 
Breamish above Linhope, and in the Linhope burn, near 
Linhope House. At the Tathey crags, near Threestone- 
burn House, the Andesite is traversed by numerous 
veins of granite, and has been considerably altered by 
it. The andesite, thus altered, has a slightly more 
lustrous and less compact appearance than the lavas 
further from the granite, owing to the re-crystallisation 
of the groundmass and the development of secondary 
biotite. 
[These altered lavas have been described in a paper by 
myself in the eighth volume of the "Transactions of 
the Edinburgh Geological Society."] 
Small crystals of tourmaline, usually only to be detected 
with the microscope, are often found in the marginal 
varieties of the granite, in some of the more acid dykes 
and veins, and in fault-breccias in the granite area. 
(3.) The Dykes and Sills. These may be divided into two 
classes :— (a) the Porphyrites, and {h) the Quartz-felsites. 
I use the term porphyrite not in its older significance 
as referring to altered (decomposed) andesite, but as 
signifying an intrusive rock representing the dyke-phase 
of the andesitic magma, and bearing the same relation 
to the andersites as the liparites and quartz-felsites do 
to the rhyolites. 
The dykes and sills of both classes are found cutting both 
the andesites and the granite. The porphyrites are by 
far the more numerous of the two. Petrologically they 
may be described as augite-biotite-porphyrites, containing 
plagioclase as the dominant felspar, while augite and 
biotite in varying proportions constitute the ferro- 
magnesian elements. In the field the rocks vary in 
colour from purple to brick-red, and the porphyritic 
felspars, and frequently also the biotite, are generally 
conspicuous in a hand specimen. They are less affected, 
as a rule, by w^eathering agencies than the lavas, and so 
