Xll INTRODUCTION. 



With the exception of two small portions of chalk at its edge, the 

 whole of Middlesex is included in the great tertiary district which 

 has been sometimes termed the London basin. 



The Chalk forms the western slope of the high ground about 

 Ilareiield, extending in a narrow slip from the northern boundary of 

 the count)' to Bungers Hill. There is also a piece about a mile long 

 forming the bottom of a small valley between Warren Gate aud South 

 ]Mims. Near Pinner the chalk is reached by a shaft about forty feet 

 deep, by means of which it is worked for lime-burning. 



Lower Eocene. — The Thanet Sands, the lowest of the tertiary 

 series, is nowhere exposed at the surface, and is only found in making 

 deep wells. It is important as a water-bearing bed. Under London 

 it is from 13 to 44 feet thick, but thins out westward. 



The Wouhoich and Reading beds of Prestwich (Plastic Clays of 

 other writers) consist of mottled claj's aud sands with rolled flints 

 from the chalk, and are remarkable for presenting, unlike other Eocene 

 deposits, identical characters in the tertiary basins of London, 

 Ilanipsliire, and Paris. 



' On the eastern side of the Colne, the Reading beds rise from 

 beneath the alluvium of the river at a point nearly two miles north 

 of Uxbridge, being followed a short way further north by the chalk.' 

 (Tniifaker, p. 16.) Near Harefield both mottled plastic clay and 

 sand have been found. Sands belono-ina; to these beds are also found 

 on both sides of the small valley between Mims Wood and South 

 Mims. 



There are two inliers of these beds in the county surroimded by 

 London clay, the denudation of which has laid them bare in conse- 

 quence of elevation produced by some local disturbance. The most 

 westerly of these is on the north-east and east of Ruislip Wood, and, 

 with an area of about half a square mile, forms ' in great part a rough 

 common-land, quite different in character from the surroundmg clay- 

 hills. It seems to be bounded on the south by the large reservoir (p. 

 28).* Just south of the Green Lane are some old sand pits. Along 

 the northern edge of the wood, plastic clay has been got, and other parts 

 yielded sand. In some places the beds are hidden b^- drifted loam. 

 The Pinner inlier is of a more irregular shape, and rather larger. 

 It follows the course of the brook from Wood Hall southwards to 

 Pinner Grove. At the lime-kiln south of the first place, the chalk is 

 reached by the shaft before mentioned, tlirough the Woolwich beds. 

 Close to the shaft is a sand pit (p. 29). A very small inlier exists 



* The pages refer to Whitaker's Memoir. 



