278 



We have now to consider the volcanic representatives of the inter- 

 mediate group. We adopt the term andesite as the general name for this 

 division. Where the rocks have been subjected to widespread and exten- 

 sive alteration, as is especially the case with those of pre-Tertiary age, 

 the term porphyrite will be frequently employed. 



Typical andesites of Tertiary age are not known to occur in the British 

 Isles. The rocks described in the previous chapter as andesitic dolerites 

 and basalts might be called augite-andesites ; but they are very different in 

 character from the typical andesites of Hungary, the Andes and the 

 Rocky Mountain region. M. BREON describes, under the term augite- 

 andesite (ande'site-augitique a labrador), certain rocks from Iceland which 

 possess affinities with our andesitic dolerites. They are dark in colour and 

 are composed essentially of microlites of felspar and augite with occasional 

 porphyritic crystals of labradorite. These rocks succeed the olivine- 

 dolerites. It is probable that the allied rocks which occur as dykes in the 

 North of England, South of Scotland and Arran bear the same relation to 

 the great sheets of olivine-dolerite occurring in the North of Ireland and 

 West of Scotland. 



Lavas and tuffs of intermediate composition are very common in 

 certain Palaeozoic areas. Those of the Cheviot district have been described 

 to some extent by the present writer w and by Dr. PETERSEN.^) The least 

 altered rock is an enstatite-andesite, or to speak more correctly an 

 enstatite-augite-andesite for both rhombic and monoclinic pyroxenes 

 are present. 



Tliis rock was termed pitchstone-porphyrite by the earlier observers. 

 It may be observed In sitd about a quarter of a mile above Windy Haugh ; 

 near Carl Croft on the Coquet ; and near Fairhaugh on the Usway. It 

 occurs abundantly as boulders in the Alwin. The best known exposure 

 is in the north-eastern portion of the district near the village of Yetholm. 

 The rock is black and possesses in its typical form a well-marked resinous 

 lustre. It is porphyritic in texture ; the porphyritic constituent being a 

 fresh glassy looking triclinic felspar. Veins of opal and chalcedony, mostly 

 stained red by ferric oxide, traverse the rock in various directions. The 

 impregnation of trachytes and andesites with secondary silica in the form of 

 opal has especially engaged the attention of Prof. VON LASAULX < 3 > and Dr. 

 HATCH. A variety of the so-called liparite of Rosenau in the Sieben-gebirge 

 containing 69'45 of SiO- has been shown b}^ the former observer to be a 

 normal sanidine-trachyte impregnated with secondary silica: 18-91 p. c. of 

 the rock being soluble in a 40 p. c. solution of caustic potash. Dr. HATCH** 

 has proved that some of the andesites of South America have been similarly 



(1) G.M. Decade II. Vol. X. (1883), pp. 100, 145 and 252. 



(2) MikrOskopische und chemische Untersuchungen am Enstatit-porphyrit aus den 

 Cheviot Hills Inaug. Diss. Kiel, 1884. 



(3) Sitzungsber. der niederrherm Ges. Bonn 1885, p. 119. 



(4) T.M.M. Neue Folge. Band VII., p. 332. The above solution was used by Dr. HATCH 

 in the proportion of 20 c.m. to 1 gr. of the rock powder. The powder was treated over the water 

 bath for one hour with repeated stirring. An increase in the length of the time of treatment 

 did not appreciably increase the amount of matter dissolved. 



