THE MINING DISTRICTS 0¥ CORNWALL. 



325 



workings of the mine has lost three fourths of its magnesium *. 

 Similar effects appear to have been produced at Huel Seton, where 

 the amount of magnesia in the rock bounding the great cross course, 

 which is traversed by the modified sea-water constituting the well- 

 known "lithia spring," is twice as large as it is in the normal killas 

 of the locality. The magnesium of the sea-water has, in this case, 

 almost entirely disappeared f . With respect to the " talco-micaceous " 

 and hornblendic slates of the Lizard, De la Beche remarks : — " The 

 hornblende slate seems intimately connected with the talco-micaceous 

 slates above noticed, as may be seen near Poltreath, on the west of 

 the Lizard town. It supports the great mass of Lizard serpentine, 

 with an apparent passage of the one into the other in many places''^. 



Pre-Granitic Eruptive Bocks. 



In different parts of Cornwall, and particularly in the more nor- 

 thern portions of the county, trappean rocks occur interstratified with 

 the ordinary killas or clay-slates. They are sometimes hard, crys- 

 talline, and vesicular ; while at others they have a schistose structure, 

 and are less coherent than the non-foliated varieties. The trap- 

 rocks and ash-beds so graduate into the slates, that the change is 

 almost imperceptible ; and they have been apparently erupted, the 

 former in a molten state, and the second in the form of ashes, during 

 the period when the mud now constituting the slates was being 

 deposited. 



Rocks of this character are described by De la Bcche as forming 

 continuous beds of considerable thickness, particularly in the direc- 

 tion of Davidstow and St. Clether. These, together with other very 

 different rocks, are laid down on the Ordnance Geological Map under 

 the general name of " greenstones." 



Near the village of Trelill, in the parish of St. Kew, there is a 

 patch of greenstone, which is somewhat extensively quarried for 

 road-material. It is an exceedingly hard and tough stone, without 

 any trace of lamination, of a dark greyish green colour, spotted with 

 white. The white spots chiefly consist of calcite, which fills vesicles 

 occurring plentifully in the original rock. It likewise contains a 

 small proportion of some zeolitic mineral, with which a few of the 

 smaller cavities have become filled. It is to be remarked also that 

 some of the larger vacuities in this rock have been filled with milky 

 quartz. 



A good geological section is not to be obtained in this locality ; 

 but the trap is known to occupy an area of some fifteen to twenty 

 acres extent. From the number and arrangement of the numerous 

 vacuities which it originally contained, it is evident that this rock 

 did not become solidified under the influence of any considerable 

 amount of pressure, but that, on the contrary, it constitutes a por- 



* Phil. Mag. 1871, vol. xli. p. 95. Calculated from the proportions of the 

 other constituents. 

 t Phil. Mag. 1873, vol. xlvi. p. 31. 

 J ' Geology of Cornwall, Devon, and West Somerset,' p. 30. 



