352 T. M. Hall — Extent of Geological Areas. 



stumps, and leaves matted together. These vegetable remains 

 are natives of the country. Of 24 stumps measured, the greatest 

 circumference was 12 feet, the trees being from 18 to 20 feet in 

 length. Horns of ruminants and the fruit of trees have also been 

 found. ^ 



(b) Underlying the peat is a thin bed of blue clay in which the 

 trees probably had root. 



(c) Chocolate-coloured Boulder-clay, with pebbles and scratched 

 stones, many being of the fundamental Magnesian limestone and 

 contiguous Trias rocks. Boulders of granite, basalt and nodules of 

 gypsum are also found. Near the base of the clay section is a thin 

 sand bed 10 inches. The "boys" left by the workmen to measure 

 the excavations, in another part of the Slake, showed 



Sea sand 2 feet 10 inches. 



Shell bed, cockles, mussels ... ,, 4 ,, 



Sand ... ... ... ... 1 „ 6 „ 



Eed clay with pebbles. 



The Slake appears to have been a hollow eroded out of slightly 

 inclined Boulder-clay, constituting a fresh-water lake, which merged 

 gradually into a bog or morass, over which in time the sea broke, 

 forming a bay which finally silted up. This peat is part of a 

 so-called sub-marine forest which has long been known to exist 

 between Hartlepool and Seaton Carew. After a hurricane has 

 blown off the superficial covering of sand, the peat can be traced 

 in patches along the shore from Hartlepool as far as the Trias rocks 

 at Seaton. 



IV. — On a Method of Estimating the Extent of Geological 



Areas. 

 By TowNSHEND M. Hall, F.G.S. 



FOE many purposes it is often desirable to form some approximate 

 estimate of the area occupied by a particular deposit, and in 

 working out the topography of any county or district (especially for 

 agricultural purposes), it is frequently necessary to ascertain the 

 relative extent covered by the various formations. This information, 

 in every instance which has come under my knowledge, has been 

 obtained by measuring on the map with a rule, and entering the 

 results, according to the scale, as so many square miles of surface. 

 Nothing being more uncommon in nature than a straight line of 

 boundary, every one who has made the attempt must be aware how 

 impossible it is to reduce with any degree of accuracy to a geometric 

 figure such lines as those to be found on most geological maps, and 

 having recently had occasion to make a calculation of this nature, 

 for Devonshire, I wish to describe a very simple method which I 

 adopted with success. 



As an ordinary map is supposed to be a counterpart on a certain 

 reduced scale of the boundaries of the district it represents, so a 

 geological map would be presumed to show the exact limits of each 



1 Young and Bird, in their " Geological Survey," mention having found in this 

 peat at Hartlepool, the remains of insects, particularly the elytra of beetles. 



