Vol. 50.] IGNEOUS ORIGIN ON DARTMOOR. 34)5 



No. 12. That part of the hand-specimen which is mounted on the 

 slide appears to be one of the slaty inclusions in one of these ash-beds. 

 It is of micro-granular but homogeneous structure, shows no distinc- 

 tive marks of igneous origin, and much resembles a slice I had 

 made from a slaty inclusion in a lava from this part of Sourton Tors. 



No. 13. This is a rock of similar character to Nos. 10 and 11, 

 but the included fragments have become sparser and smaller, and 

 seem to have been, in most cases, nearly melted down and assimilated 

 by the igneous rock. Its rhyolitic character has disappeared. The 

 groundmass is finely crystalline-granular, with microlites of felspar 

 dotted about in it here and there. Sometimes these microlites are 

 sharply defined, and have forked ends ; at other times they are 

 somewhat ragged and irregular at their side edges : all of them have 

 straight extinction. In this groundmass are embedded numerous 

 porphyritic quartzes and felspars, some of which are idiomorphic. 

 The felspars have straight extinction, and are orthoclase ; some 

 show a somewhat irregular polysynthetic twinning. One large 

 felspar of this character, the irregular twins of which extinguish 

 symmetrically at 18° to 19°, contains endo-idiomorphic crystals of 

 felspar with binary twins, one of which is certainly orthoclase. 



This slice contains a mineral of exactly the same habit and general 

 character as the anthophyllite of No. 8. Some of it has a pale- 

 yellow tint, and some of the prisms are slightly dichroic. The ex- 

 tinction varies from 0° to 15°. The mineral in this case must be 

 actinolite : a slight rise in the percentage of iron has probably 

 determined the change in the optical character and species of the 

 mineral. The slice moreover contains magnetite, ferrite, and arnica, 

 which varies from buff to red in colour. The last-named mineral 

 also fills the vesicles in one of the included fragments of lava, as it 

 often does vesicles in other specimens : it dissolves readily in hot 

 dilute hydrochloric acid, and the solution yields a little lime and 

 alumina, much iron, and magnesia in perceptible amount. It also 

 reacts for potash. This mica must be lepidomelane or an allied 

 species. 



Felsites. 



It will be seen from the above account of the volcanic tuffs of 

 Sourton Tors that the pyroclastic rocks are intercalated with felsites 

 containing numerous fragments of ejected material. It is not 

 therefore a matter for surprise to find the pure felsites (described 

 below) occurring among these beds. 



No. 14 1136. Sp. Gr. 276. From place marked 6 on Map. 



„ 15 1223. „ 2-74. „ 8 „ 



.„ 16 1211. „ 2-72. „ 1 „ 



„ 17 1216. „ 2-72. „ 6 „ 



Nos. 14 and 17 are from the same locality, and are identically 

 the same rocks. Macroscopically considered, they are perfectly com- 

 pact in texture, with blebs of quartz and very minute grains of 

 felspar visible here and there. They are also traversed b) r fine 



