1^23.] GeologkalTmnsactioiiSQfCofmvaUy Vol. 11. 49 



XV. On the Mineral FroductionSy and the Geology of the 

 Parish of St. Just. By Joseph Carne, Esq. FRS. &c. MGSC. 



The space to which our analysis must be confined obHges us 

 to pass over Mr. Game's enumeration of the minerals which have 

 been discovered in this district, together with the greater part 

 of his description of the peculiar geological facts observed in it : 

 the only sections of the latter for which we have room are the 

 following: 



" Floors, or Horizontal Beds. — St. Just abounds in floors of 

 tin, more thaii any other part of Cornwall. 



" In that part of the tenement of Trewellard which is in a 

 slate country, some tin floors have been wrought near the sur- 

 face ; the deepest is only seven fathoms below it : they were 

 from one to two feet in thickness, and perhaps twenty feet in 

 diameter;* they occurred at the junction of several tin lodes. 



In Huel St. Just, a mass of tin ore, of a very singular nature^ 

 was discovered some years ago, which appears to belong to the 

 floor formation. It first appeared at the depth of 17 fathoms 

 under the sea, and has been followed downwards about 10 

 fathoms. It was seven or eight feet in diameter. At the top, 

 it was on the south-western side of the tin lode; but it inclined 

 in a very small degree, until it was almost wholly on the north- 

 eastern side of the lode. The cavity in which \t was found, had 

 the appearance (after the tin was taken away) 6f a large under- 

 lying shaft, closed at the top. The most remarkable circum 

 stance, however, relates to the state in which the tin ore was- 

 found ; instead of being in a solid body, as is usual in floors, it 

 appeared (as the miners termed it, from whom I received the 

 account) hke a heap of at tie ^ or rubbish; just as if it had been 

 thrown in that state into the cavity. The fragments were not 

 rounded, but had all the appearance of the broken tinstone, which 

 is generally seen on the surface of tin mines. The top of this 

 mass of ore was about three feet below the granite top of the 

 cavity, as if it had sunk by contraction or pressure. One of the 

 miners told me that he found sufficient space between the gra- 

 nite covering and the ore, to sit upright on the latter. In its 

 present deepest part, it is not so wide as it was at a higher level ; 

 but it is more compact. The tin ore, as raised from this cavity, 

 contained, according to the miners' mode of calculation, from 

 700 to 1000 of tin to every 100 sacks. This floor, although of 

 far greater thickness than any other which has yet been disco- 

 vered, does not appear to be the result of the union of several 

 lodes, for no such union takes place near it. Only one lode 

 has been found connected with it, which, although perfectly 

 distinct in the granite, both south-east and north-west of the 

 floor, appears to lose its individual character, and to form one 



• " It must not be supposed from this description that the floors are round: on the 

 contrary, they are frequently very irregular, but theif surface is about as large as would 

 be comprised in a circle of 20 feet diameter." 



New Series, vol. vi. e 



