Geological Survey of the Island of Jersey, 36 i 



The comparative durability of the substances of the two 

 rocks is singularly shewn by the invariable failure of the 

 argillaceous schist before the action of the sea, having chasms 

 and channels often of considerable depth ; as is the case at 

 the little insulated headland n, in Portelet bay, whereby a 

 broad and deep dyke has been formed: the vein has also 

 forked down at this point. At o, there is another similar 

 chasm, at least 50 feet deep, and 20 wide ; the walls of sienite 

 are quite vertical. 



So complete has been the removal of the argillaceous schist 

 in these cases, that caverns 20 or 30 feet deep have been 

 worn out in the cliffs where these veins first emerge ; the 



ceiling of these is actually composed of the debris of the 

 vallies above, shewing clearly a more daring hand than that of 

 man in their excavation. 



The angle of inclination of the strata varies from 15° to 60°, 

 dipping generally west of north. 



21 # exemplifies the nature of the rock from the centre, 

 eastward, and 22, 22a, 22 6, 23 from the centre, westward; 

 these specimens present all the prominent points of character 

 in the argillaceous schist, as it changes from a mere shale to 

 the subcrystalline granular and extreme variety (found mostly 

 near St. Aubin's) that verges closely on grauwacke. I have, 

 also, met with small specimens of the roofing slate at Greve- 

 de-Lecq. In the St. Peter's valley, the new road, winding 

 along the bottom, finely exhibits the structure of this rock, as 

 it crosses the strata in every direction. 



That of which 21 is a specimen, crumbles and shales away 



* The numbers refer to specimens sent by the author to the Museum 

 of the Plymouth Institution, 



