Geology of the Neighbourhood of Weymouth, S^c. 5 



don : and at the summit of Black Down, six miles south-west of Dorchester, 

 which forms the highest hill within our district, we have a considerable series 

 of beds of similar pebbles, sand, brick earth, and plastic clay*, analogous to 

 the outlying fragments of the plastic clay formation which crown many of the 

 highest summits of the downs of Wiltshire and Hampshire, and are described 

 in Dr. Buckland's paper on Valleys of Elevation f. 



Upon the high table land of chalk, also, on which this plastic clay of Black 

 Down rests, we have numerous blocks of siliceous pudding-stone, scattered 

 over the surface of the fields, and composed, like that of Hertfordshire, of a 

 congeries of chalk flints imbedded in highly indurated siliceous sand: these 

 blocks also contain occasionally, though rarely, a few angular fragments of 

 chert from the greensand formation, and a few small pebbles of white quartz. 

 The chalk flints which make up the greatest part of their substance are not 

 all rounded, as in the Hertfordshire pudding-stone, but for the most part are 

 anffular, as in the case of similar insulated blocks which occur on the hills of 

 greensand near Sidmouth ; and this angular condition deserves peculiar 

 notice, as it seems to connect them with the next deposits we are about to 

 mention, some of which may possibly be referred to the era of the plastic 

 clay formation. In two deep and steep combs excavated in chalk on the west 

 and south of Black Down Hill, viz. at Bride Bottom on the west, and at Por- 

 tisham on the south, these blocks of angular breccia are accumulated as thickly 

 as the gray-wethers in Clatford Bottom, near Kennet, on the Marlborough 

 downs : their abundance in Bride Bottom has caused it to be called the 

 Valley of Stones. This bottom forms the upper extremity of the Vale of Bredy, 

 where it contracts into a deep and narrow comb, at the head of which the 

 blocks are spread over a space of several acres, like a flock of sheep, often so 

 close together that a man may leap successively from one to another of them. 



On the south side of the plain of Black Down we have another collection of 

 huge blocks of the same breccia in the steep comb which descends the chalk 

 escarpment into the village of Portisham : they are most abundant in the 

 village itself, where they lie so thick that they partly obstruct the street, and 

 when they occur in the line of the houses the walls are built upon them : 

 their extreme hardness and bulk have hitherto prevented their removal or 

 destruction by the hand of man. In the street at West Lulworth, also, similar 



* These strata supply materials to a manufactory of bricks, tiles, and coarse pottery that is 

 established on this hill. The clay exhibits the same varieties of colour, viz. blood-red, yellow, 

 white, and black, which are so characteristic of the plastic clay formation, and are so well shown 

 in Alum Bay and at Reading. 



t Geological Transactions, Second Series, vol. ii. Part I., p. 119—130. 



