XI 
\ 
and hornblende are their principal constituents.* Near the granite 
they are mostly of green, brown, purple or violet hue; but at 
greater distances from it they are frequently of grey, *plueish- 
grey, deep blue, brownish-yellow or buff colour ; “and some of 
these, in distant parts of Cornwall, enclose organic remains.T - 
Certain slates are crystalline and cleave imperfectly, others show a 
thick-lamellar structure, many are highly fissile, and all are, more 
or less, interlaminated and veined with quartz. Intercalated 
bodies of hornblendic and felspathic rocks occur as broad bands 
of considerable range{ in some, but at isolated masses in other§ 
places. The planes of cleavage almost always dip from the 
granite, the various layers of slate thus mantle round the slopes 
of the granitic hills;|| but their laminz seldom dip so rapidly as the 
* Careful analysis of killas from various Cornish mines afforded the 
following results ;— 
Tron in 
MINES. Silica. | Alumina.] various Lime. Soda. Water. 
states. 
50:8 19:8 6°53 1:2 1195) 3°2 
IPOWOOU viele ~~ «00 \ to to to to to to 
§3°2 21 18°5 1:8 4:9 4-1 
49-2, 22: 6:5 
Polmear .ccc--s i to ts to \ 2-1 0:7 \ to 
49-3 (29-1 6-6 
20°7 4-3 1:9 3°3 
Dolcoath «+ ' 67°3 { to to to to Heil 
20°9 4-6 2-1 3-4 
32°9 16°7 15:3 4- 0°6 371 
Botallack....... to ' to to to to to 
40-2 24° 20°8 5- 3°6 11-1 
Potassa and titanic acid occurred in every case, but always in minute pro~ 
portions; magnesia also was ever present, but generally in small quantities ; 
except at Botallack where it amounted to from 6:4 to 11:6 per cent. 
Phosphoric acid was detected at Polgooth and Botallack; but the oxide 
of manganese and manganoso—manganic oxide were discovered nowhere but 
at Polgooth, and sulphur was found at Botallack only. 
Puruuies, (Joun AntHuR), Lond. Edin. and Dublin Phil: Mag: Fourth 
Series, xli., pp. 87-95. 
+ Dela Beche, Report on the Geology of Cornwall, Devon, and West 
Somerset, pp. 82, 351. Henwood, Cornwall Geol: Trans: v., pp. 125, 158; 
Viii., p. 700. Peach, Ibid, vii., p. 104. 
‘t Pryce, Mineralogia Cornubiensis, p . 76. Rogers, Cornwall Geol: Trans : 
aie pp. 218-220. Boase, Ibid, iv., pp. 208, 210, 306, 311. De la Beche, Re- 
port, pp. 79, 100. Henwood, Cornwall Geol: Trans : Vili., p. 671. 
§ Henwood, Reports of the Royal Institution of Cori naval, XXxiii., p. 39; 
Cornwall Geol: Trans : vili., p. 701. Giles, Ibid, vii., pp. 201, 205, 206. 
|| Boase, Cornwall Geol: Trans : iv., p. 365. Henwood, Ibid, v., p. 144. 
_ Von Cotta, Ore Deposits (Translated by Prime), p. 403. 
