INTRODUCTIOX. XUl 



also along the course of a small brook about a quarter of a mile west 

 of Pinner station. 



The London Clay in the tertiary district westward of London 

 consists, according- to Prestwich, of tenacious brown and bluish-grey 

 clays with layers of septaria. He also remarks that it everywhere 

 maintains throughout its mass a nearly uniform mineral character. 

 ' Its boundary line rises from below the alluvium at a spot about a 

 mile and a half north of Uxbridge ' (p. 49). East of the Colne 

 the London clay forms the greater part of the well-marked tertiary 

 escarpment extending north-east from Kickmansworth to Hatfield. 

 Some way south-east it rises into a second and higher ridge, often 

 capped with gravel (p. 96). 



Immediately at the base of the London clay, a bed of sand, never 

 more than a few feet in thickness, is commonly found (' basement bed/ 

 Prestwich). It has been observed near the Ruislip Wood inlier of 

 the Woolwich beds. The upper beds are also more sandy than the 

 general mass, but are seldom found, owing to denudation, at any 

 great distance from the overlying Bagshot beds. The Archway Road 

 at Highgate originated in an attempt to drive a tunnel through the 

 hill ; but this was abandoned on account of the sandy nature of the 

 upper beds, rendering them incapable of supporting an arch of the 

 dimensions required. (Conybeare and Phillips' Geology of England 

 and Wales, p. 25.) 



Middle Eocene. — The Loiuer Bagshot Sands are represented in 

 Middlesex by three outliers, forming the tops of Harrow, Highgate, 

 and Hampstead Hills. Mr. Whitaker gives the following account 

 of these : — 



^The Harrow is the smallest, and reaches from tlie north of the 

 church, southwards to Mount Pleasant, and is very narrow at its 

 northern end. The largest and best-marked forms all the highest 

 ground of Hampstead, and gives rise to the well-known gorse- 

 covered heath. The irregularly- winding boundary of the sand is 

 well marked both by the sharp rise of the ground, and by the water 

 thrown out from these permeable beds by the underlying water-tight 

 London clay ; * but the northern end of the outlier, in Bishop's Wood, 

 is rather hidden. The sand spreads from the western end of Lord 

 Mansfield's kitchen garden, on the road to Highgate, westward nearly 

 to the high road at Child's Hill, and southward to Rosslyn House. 

 There are many pits on the heath, and they show light-coloured 

 sand with thin layers of clay here and there : so much indeed has 



* One spring at Hampstead is chalybeate. 



