518 Notices of Memoirs — Fox ^ Somervail — Lizard Porphyrites. 



passage to the pump saturates itself taking up sufficient salt to make 

 a solution containing about 26 per cent, of salt. This continual 

 removal of salt from the surface of the rock-salt lowers it, and the 

 overlying earths either follow the diminishing surface continuously, 

 or else after remaining suspended for a time suddenly fall into the 

 cavity from which the water has extracted the salt. The brine 

 currents on their way to the pumping centres form deep valleys 

 or troughs, and the surface of the ground overlying forms a facsimile 

 of these hollows. The property on the sloping sides of the valleys 

 is pulled to pieces and destroyed; the windov^s and doors all get 

 out of form owing to the unequal sinking of the various portions of 

 the house. When, owing to the different nature of the marls and 

 the abundance of sand overlying them, a sudden sinking takes place, 

 the hole extends to the surface and swallows up anything upon 

 the surface— as a horse in a stable, barrels of beer in a cellar, or 

 w^ater butts and other utensils in a yard. The damage done to 

 property is enormous, but thus far no human life has been lost. 

 The most serious part of the matter is that the brine-pumper takes 

 not only his own salt in solution, but that of all his neighbours over 

 whose salt beds the water flows, and neither asks their consent nor 

 pays them for the salt thus obtained. Worse even than this, the 

 owner of the property overlying the brine ' run ' suffers most serious 

 damage to buildings, etc., but can obtain no compensation, because 

 amongst the number of brine-pumpers he cannot prove who is doing 

 the particular mischief complained of. This peculiar phenomenon 

 of subsidence in the salt districts is worthy of more consideration 

 than it has hitherto received from scientific men. 



IV. — On the Occurrence of Porphyritio Structure in some Eocks 

 OF THE Lizard District.^ By Howard Fox and Alex. Somervail. 



PEOF. BONNEY has described a porphyritic diabase which is 

 seen on the shore at Polpeor ; it cuts, in an intricate manner, 

 through micaceous and hornblendic schists. The authors have 

 traced this rock further, and have recognized a porphyritic structure 

 in many dykes and intrusions along the coast which cut through the 

 serpentine, and also in the darker bands of Professor Bonney's 

 'granulitic group.' 



Descriptions of these various localities were given and illustrative 

 specimens exhibited. The crystals of felspar are found to be most 

 numerous in those rocks which lie in the closest proximity to 

 the gabbros and serpentine. They have their long axes at various 

 angles, and are mostly small except at Parn Voose, Cavouga, and 

 Green Saddle. The felspathic and hornblendic lines often circle 

 round the crystals. 



Without discussing any theory as to the true nature and origin of 

 the whole of the schists, the authors think that the porphyritic 

 structure so prevalent in the dark bands of the ' granulitic group,' 

 in many of the micaceous and other rocks, as also in the later 

 intrusions cutting the serpentine, indicate an igneous origin for 

 many rocks hitherto regarded as schists. 



1 See Mr. Teall's paper, ante, pp. 484-493. 



