80 GEOLOGY OF THE ISLE OF WIGHT. 



of siliceous particles ; a mass of the white variety 

 recently broken strikingly resembles the surface 

 of lump sugar. Erratic blocks of this sandstone 

 are common on the chalk clowns of Wilts, Sussex, 

 Kent, &c, and are provincially called the Grey 

 Wethers ; they occur also in the superficial drift 

 and gravel. 



London clay. — This deposit in the neighbour- 

 hood of the metropolis forms a dark-blue stiff 

 soil, and has occasional intermixtures of green 

 and ferruginous sands and variegated clays. It 

 abounds in spheroidal nodules of indurated argil- 

 laceous limestone termed septaria,* which are 

 internally traversed by veins of spar disposed in a 

 radiated manner from the centre towards the 

 circumference, and often contain shells and other 

 organic remains. -j- The septaria are generally dis- 

 tributed in horizontal layers, and lie at unequal 

 distances from each other. Brilliant pyrites and 

 crystals of sulphate of lime abound in the clay. 



Plastic clay. — Beneath the extensive and thick 

 deposits of London clay, and interposed between 

 them and the chalk, there occur in some localities 

 strata of sand of various colours, alternating with 

 layers of bluish plastic clay; and there is very 



* The septaria, when calcined and ground, constitute the composition 

 called Roman a >ru ni 



t Medals ol' Creation, p. liT.'i. 



