152 DREDGINGS OF THE MARINE BIOLOGICAL ASSOCIATION : 



Bolivina, Cristellaria, and Lagena. There are occasional black and 

 dark brown specks. In all except its liardness the rock is distinctly a 

 chalk. (Plate IX, figs. 1 and 2.) 



Mr. D. J. Matthews has kindly made an analysis of a portion of the 

 specimen above described, and returns : — 



Silica (SiOo) 0*70 per cent. 

 Calcium Carbonate (CaCOg) 94-05 per cent. 

 Magnesium Carbonate (MgCOg) 1*56 „ 

 Phosphorous pentoxide (PoO,) 0'66 „ 

 A little Iron and Aluminia. 



Alkalies not tested for. 



Prom the above it will be seen that the rock is both phosphatic and 

 dolomitic. 



Hard Yelloiv Chalk was also taken at the following stations, among 

 others. The list is not quite complete : — 



M. 31, M. 36, M. 33, M. 35, M. 15, M. 25, M. 17, M. 41, M. 58, and M. 67. 



FLINT. 



Flint is quite the commonest rock if the whole area covered by the 

 dredgings is considered. It would be difficult to assert positively that 

 it is anywhere entirely absent from the stony grounds. A. 90, about 

 four miles toward Plymouth from the Eddystone, is all Devonian, but 

 this was on a sandy ground. 



Many of the flints are very unlike, in external appearance, any 

 usually seen on land.^ The unaltered mineral is very frequently 

 black, but occurs only in the heart of the pebbles. A solvent 

 action, not necessarily entirely marine, has removed a portion of 

 the silica for some depth from the surface of the stone, and has 

 left a white coherent gritty substance, which is sometimes soft 

 enough to mark on a blackboard, sometimes quite hard. The pro- 

 gress of this alteration can frequently be traced in the section of 

 a broken stone. Pebbles of some inches in thickness are often 

 so far affected that a mere remnant, a patch of perhaps half an inch 

 diameter, will be left in the centre to show what the former condition 

 was. In some even of the large pebbles no unaltered flint remains. 

 It seemed desirable to ascertain what proportion of the original 

 mineral has been removed by this solution, and as an approximation 

 the following method was adopted: M. 15. A piece of thoroughly 

 altered flint, from a stone which showed some remnant of black flint 



^ Exceptionally, as, for instance, on Hardown Hill and Annis Knob, near Lyme Regis, 

 chert and black Upper Chalk flint are fonnd in much the condition here described, but 

 the presence of carbonate of lime has not been re])orted, although probably existing much 

 as in these specimens. 



