1815,] Transactions of the Geological Society, Vol. II. 449 



Common salt 96*48 



Sulphate of lime 1 'b'S 



Sulphate of soda 1 "^2 



Muriate of magnesia 0*07 



1 00-00 



I doubt whether Mr. Horner's criterion of the absence of sul- 

 phate of magnesia be aiifficient. After the removal of the lime he 

 added ammonia without obtaining any precipitate. I think it pro- 

 bable, that a small quantity of sulphate of magnesia, held in solu- 

 tion in a complicated saline liquid, would not precipitate with 

 ammonia, in consequence of the solubility of the triple salt formed. 

 But I have not tried the experiment. 



VI. Off the rtins of Cuniwall. By William Philips, M. G. S.— 

 The ore in Cornwall occurs commonly in veins which have an east 

 and west direction, and are called lodes. These are traversed by 

 north and south veins called aoss courses j and sometimes by north- 

 east and south-west veins, called contres. The relative antiquity 

 of these veins appears in general to be in the order in which they 

 liave been named. The lodes are the most important ; they contain 

 tin and copper ore. The tin ore is commonly towards the upper 

 jrart, and tJie cojjper towards the lower part of the lode. No one 

 of these lodes has been traced further than two miles; yet there 

 can scarcely be a doubt that they traverse the whole county, and 

 even pass through Devonshire, till they lose themselves in the 

 channel. No vein has been followed to its greatest depth. Their 

 most common breadth is from one to three feet. Tl'.ey are very 

 rarely 30 feet wide. When a cross course cuts a lode it generally 

 changes its direction, or heaves It, as the miners term it. The cen- 

 tres do the same thing to both the others. The cross courses are 

 usually filled with quartz j though sometimes likewise they con- 

 tain ore. 



There is a low range of hilly country that traverses Cornwall 

 •just on the south of the principal veins. This range, as far as I 

 observed, was granite. On the north and south side this granite is 

 covered by clay-slate, which goes to an unknown depth. The 

 veins usually traverse the clay-slate, and when they are followed 

 far enough, they are found likewise entering the granite, which 

 spreads itself below the clay-i;late. Uoru'-stone porphyry likewise 

 passes through the clay-slate, but whether in beds or veins has not 

 been deternilned. It is callerl clnan by the miners. The top of a 

 lode is usually called gossan, being filled with a yellow ochrey 

 matter to which (hat name is given ; and lodes have received the 

 following names, according to the prevalent substance with which 

 they are filled: — GovAV/ff^, when abounding in gossan: s/xiin/, 

 when abounding in quartz, oi fluor spar, both of which arc called 

 spar, in Coinwiill. The latter commonly green spar or sugar spar: 

 inumildai, when abounding In iron pyrite=: : fjiachy. when abound- 

 ing in chlorite : Jldiickany, when filled chietiy with u kind of tUy: 

 Vol.. V. N^ VI. 2 F 



