254 LIAS OF ENGLAND AND WALES : 
UPPER LLAS. 
LOCAL DETAILS. 
Dorsetshire. 
In the cliff-section at Thorncombe Beacon, and on top. of the 
adjoining Down cliff, we can trace about 70 feet of blue micaceous 
marly and sandy shale. The beds arc inaccessible, but they arc 
seen to merge upwards into the Midford Sands, which are shown 
to a thickness of about 100 feet at the Beacon. At the base of 
this mass of blue shale, is the thin layer of pale argillaceous lime- 
stone (Basement Bed of Upper Lias) welded to the top of the 
Middle Lias, and before described as the Junction-bed. These 
beds are also seen at Allington near Bridport, (See Fig. 41, p. 52.) 
Owing to the thinness of the rock-beds at the junction of 
Middle and Upper Lias there are no quarries along this horizon 
by Netherbury and Burstock, between Bridport and tlminster, 
and the Basement-beds of the Upper Lias are not to be seen. 
Moreover the shales of the Upper Lias nre often very sandy 
in character, and much like the beds immediately under the Marl- 
stone Rock-bed ; we find no uniform mass of dark clays and shales, 
such as occur in Gloucestershire, and further north ; so that 
having only occasional sections in the deep lanes, it is very difficult 
to mark the boundaries of the formations, and over the greater 
part of the country between Bridport and Bath the Upper Lias 
was omitted from the Geological Survey Map. 
We have in fact a mass of clay or shale, which merges upwards 
into the Midford Sands, and is not in this area separated by any 
prominent rock- bands from the soft sandy and shaly beds of the 
Middle Lias. The presence of the formation is indicated by the 
clayey nature of the ground, and could no doubt be traced con- 
tinuously with the aid of the 6-inch maps, but in isolated sections 
it is not possible, in the absence of stratigraphical and palaeontolo- 
gical evidence, to determine the age of the bed.''. Thus the age of 
the laminated clays and sands at Chalkway between Winsham 
and Cricket St. Thomas was not to be determined. (See p. 202.) 
Mr. S. S. Buckman has recently obtained from the upper part 
of the blue shale at the Down cliffs (about 12 feet or more from 
the top) several Ammonites, one species of which he was enabled 
to determine as A. radians, a form found also in the yellow sands 
above.* This fact agrees with the stratigraphical evidence, of the 
intimate connexion between the clays and sands, and suggests that 
possibly the whole of the Upper Lias shale of Dorsetshire may 
belong to the zone of A. jurensis. No fossils are recorded from 
the lower portions (the main mass) of the shales. It must be borne 
in mind that the range of individual species of Ammonites may be 
* Quart. Journ. Geol. Soc., vol. xlvi. p. 519 ; and H. B. W., Ibid., p. 521. 
