COAST OF THE UNDERCLIFF. 251 



usual smooth verdant character of the chalk is 

 resumed, and the hills rise to the height of 800 

 feet, sending oft' to the northward the lofty range 

 of Shanklin Down, and terminating in Dunnose 

 Point.* The siliceous nodules in the&e strata are 

 always found entire ; presenting in this respect a 

 striking contrast to the shattered flints in the ver- 

 tical strata that form the opposite range at Brading 

 Down (see PL XIII.). 



Coast of the Undercliff. — The district of 

 the Undercliff, from the western extremity of St. 

 Catherine 's-hill to Bonchurch, is about six miles ; 

 and the road from the west passes along an irre- 



* Deviation of the plumb-line at Shanklin Doivn. — During the progress of 

 the Ordnance survey, a deviation of the plumb-line from its mean direction 

 was observed in the neighbourhood of Shanklin Down, by Col. Colby, the 

 Director of the Trigonometrical Survey; a considerable attraction of the 

 plumb-line to the south of the Dunnose station having been unequivocally 

 determined, and which is supposed to depend on the intensity of the attrac- 

 tion of the hill. In alluding to this remarkable phenomenon Sir Roderick 

 Murchison (in his address at the meeting of the British Association of 

 Science at Southampton, September 184C) observed, " that it is a matter of 

 surprise that this, comparatively, low chalk range in the Isle of Wight should 

 attract, in one parallel at least, with more than half the intensity of the high 

 and crystalline mountain of Schehallien in the Highlands of Scotland. a 

 Can this be explained by the peculiar structure and distribution of the ridge 

 of upheaved strata which runs as a back-bone from east to west through the 

 Island? or is it to be referred to the effect of dense masses of plutonic rock 

 ranging beneath the surface along the line of displacement of the deposits ?" 



a In the year 1774, from astronomical observations made on the mountain of 

 Schehallien by Dr. Maskelyne, it was proved that a sensible disturbance was 

 produced in the direction of the plumh-line by the attraction of the dense 

 masses of granular quartz and limestone, and mica and hornblend slate, of 

 which that mountain is composed. 



P 



