69 



weathered ; the first effect of weathering is decalcification, 

 by which the clay and stones are left without the com- 

 minuted chalk, and then by the wash of rain and snow the 

 fine clay and sand pass away and the larger stones remain. 



The chalk in the Boulder clay is often found covered 

 with glacial striae, comminuted chalk is abundant. The chalk 

 is usually much hardened ; either from pressure, or its 

 subjection to salt water, or to parts being infiltrated with 

 the lime dissolved from other parts. The clay is usually 

 dark coloured and blue. By weathering and oxidation it 

 becomes greenish and yellow, at the same time losing 

 much of the chalk which has been dissolved out. After it 

 had been subjected to subsequent glacial kneading, and 

 forced as trail down the valley sides, some of the re- 

 maining blocks of chalk may be found comminuted in the 

 altered clay, for I do not consider the decalcified clay of the 

 valleys to be in the original place, or to belong to the 

 original species of the deposit, and believe that the clay lay 

 on the upland plain through which the valleys have been 

 cut, and down whose sides in varying states it has since 

 made its way. 



The Boulder Clay has not been found in sihl south of 

 the Thames, but there is no reason that it should not, that 

 I know of. The most southerly point at which it is found 

 in Essex is due Northward of Shooters Hill, and the high 

 ground to the Eastward of it ; and I would call attention 

 to the fact th^t on the patches of London clay on Bostol 

 Heath and on the spurs of that tenacious clay which extend 

 from Shooters Hill lying above 200 feet O.D., may be 

 found a notable quantity of erratics in the form of Quartz, 

 massive, saccharoid, and crystalline, Quartzites, Hard 

 Sandstones and a few other rocks in small pieces re- 

 sembling those of Essex, though of course these may be 

 the remains of its subjacent deposit the glacial gravel. 



But on Northumberland Heath may be found here and 

 there, clay, blue and yellow like altered London clay or 

 Boulder clay, which I have traced in a trail of later 



