JURA. GEOLOGY. 



are very abundant, and they are also conspicuous for their 

 magnitude. 



One remarkable rock still occurs on this side of the 



island, offering an appearance somewhat anomalous, and 



at a distance, very deceptive. It consists of a graywacke 



basis, or a paste of clay slate and quartz, containing 



distinct and large particles of quartz and felspar which 



bear marks of irregular crystallization; the felspar in 



particular often presenting its well known angular sections 



where the rock is broken. At a very short distance 



from the eye it can not be distinguished from a porphyry, 



and for such it might readily be mistaken on a cursory 



examination. There can however be no doubt respecting 



its real place among the schistose rocks ; particularly as 



approximations to it are not unfrequent among the 



schists of the adjoining hills. It is also disposed in 



beds regularly alternating with the others, while in 



many instances it loses the porphyritic aspect and 



subsides into an ordinary graywacke. Respecting the 



geographical positions relatively occupied by these several 



substances, it is unnecessary to enter into verbal details ; 



nor indeed could they be rendered intelligible, from 



their intricate nature and from the dispersed state of 



the population of Jura, which affords few names of 



places adapted for the purpose of reference. The 



reader must on this subject consult the map, where the 



general positions are indicated as nearly as the state of 



things permits. 



It is now necessary to return to the west side of 

 Jura, which remains undescribed. There is nothing indeed 

 to be said on this subject which would not be a repetition 

 of the account of the central line of the island. The 

 quartz rock here extends from the mountain ridge to 

 the shore ; commonly terminating in low cliffs, which 

 stretch from the western entrance of the Coryvrechan 

 to that of the Sound of Isla. In these the strati- 

 fication is distinctly visible, but this side of the island 



