18 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV* 20, 



6. That on the eastern side of the basin the beds of fine material 

 are more developed than on the western side. 



7. And, lastly, that the various beds run in the direction of, and 

 seem to point to, the Eiver Bovey as the source from whence they 

 were derived. 



The author then considers the probable conditions of a lake of the 

 size of the Bovey basin, elongate, but contracted in the middle, fed 

 by a rapid river entering the lake at its upper end, and having its 

 tributaries in hills clothed with forest-trees, and consisting of de- 

 composing granite, such as is seen at present on the south-western 

 slopes of Dartmoor, and at the China- clay- works of St. Austell and 

 St. Stephen's, Cornwall, where the felspar of the granite has decom- 

 posed into a soft white powder, and the quartz and mica form loose 

 sand and gravel of all degrees of size, for a depth, in some places, of 

 more than 40 fathoms. 



The materials brought by the river to the lake would (the author 

 states) mainly consist of — first, clays of dififerent degrees of fineness, 

 derived from the decomposed felspar ; secondly, earthy matter, from 

 the vegetable mould ; thirdly, siliceous sand and gravel, of all de- 

 grees of size ; fourthly, vegetable matter, forest-trees and plants of 

 various kinds, from the river, in time of flood or otherwise, under -^ 

 mining its soft banks clothed with vegetation ; and, lastly, stones 

 and boulders of various kinds. 



The particular plan of deposition, and often redeposition, of these 

 materials is then described by the author, and illustrated by a 

 diagram-plan ; and he remarks that the various strata, consequent 

 on the fluctuating quantity of water discharged by the river, would 

 not be deposited horizontally over the bottom of the lake, but would 

 incline more or less from the sides towards the centre, or towards 

 the current, the degree of inclination being regulated chiefly by the 

 strength of the current. Where the lake became very narrow, the 

 beds of sediment would be thin, and the dip great ; and where the 

 lake was wide, the dip would be comparatively small ; the dip being 

 probably caused by less material being allowed to permanently fix 

 itself at the centre than at the sides ; therefore the beds would have 

 a tendency to thicken near the surface, and thin out below, causing 

 the dip to increase towards the centre of the basin. The dip, too, 

 would not be of the same angle throughout, but would be less 

 towards the bottom ; the section of such beds assuming a slightly 

 concave form. 



Thus the lake would go on. filling with sediment, the coarse irre- 

 gular deposit of the delta advancing downwards, overlying the more 

 regular parallel beds of the fine material beneath : the materials of 

 the delta would be very thicldy deposited towards the centre, and 

 more thinly where the beds of fine material approach the surface, 

 and it would thus form a coarse unstratified *' Head," overlying the 

 finer stratified deposits. 



Mr. Key observes that the strata of the Bovcy basin, on com- 

 parison, will be found to comj^ly in every material circumstance with 



