part 4] PLIOCENE deposits of CORNWALL. 358 



of 480 feet upon an eroded granite-surface, and is of \evy obscure 

 origin. Beyond the doubtful topographical evidence as to its 

 alleged Pliocene age, there is little whereb}" to reconcile its sug- 

 gested relationship to the St. Agnes, St. Erth, or Crousa- Common 

 deposits.^ 



III. Methods of Investioation. 



In all cases the individual outcrops of the deposits were sampled 

 both laterall}' and vertically : in the first instance, suitable ex- 

 posures were located along the strike of the beds, from which 

 samples were taken at regular intervals if outcrops allowed ; in the 

 second case all the principal quarry- sections at each locality Avere 

 visited, and the sands sampled from top to bottom at intervals 

 depending on the lithological variations noted in the beds. As an 

 example of this method of procedure we may take the St. Erth 

 occurrence — in a lateral distance of some 800 yards measured 

 along the strike of the beds, four samples were taken at approxi- 

 mately equal intervals of 200 yards, the material being furnished 

 by stream, hedgebank, and quarry. For sampling in depth, the 

 Cornish Sand Company's pit was chosen, since it showed the 

 maximum vertical section exposed in the beds at the time. In 

 this case samples were taken from 6, 8, 14, 20, 22, and 23 feet 

 below the surface, these horizons representing the more marked 

 lithological variations in the deposit as seen in this pit. The 

 samples referred to in all instances consisted of about 200 grammes 

 of material, having regard to the average grade-size of the par- 

 ticles, about 0*2 mm. In the case of much coarser material, as 

 at Crousa Common, at least 500 grammes were taken for each 

 sample, in order to ensure a reasonable yield of ' heavy ' residue. 



The usual method of procedure for the qualitative examination 

 of the mineral constituents of the deposits was adopted ; the 

 samples were first sifted free of the larger grains and pebbles, by 

 using a 30-mesh sieve and later a 60-mesh sieve ; in this way each 

 sample was divided into three rough grades, a coarse, medium, and 

 fine. The coarse material, rejected by the 30-mesh sieve, included 

 the grains greater than approximately 0*5 mm. in diameter ; the 

 medium material, passed by the 30-mesh but rejected by the 60- 

 mesh sieve, included the grains between approximately 0*25 mm. 

 and O'O mm., while the fine material, passed by the 60-mesh, in- 

 cluded all fine sand, silt, and clay : that is, particles less than 

 0*25 mm. in diameter. It is desirable to emphasize that such 

 grading is onl}?" of a very approximate character, and is used solely 

 for convenience in qualitative work, but not for accurate mechanical 

 anal3^sis, which must be obtained by the ordinary methods of 

 elutriation ; it is of use, however, in giving a rough impression 

 of the relative proportions of the particles between the chosen 

 limits. 



1 Ihid. p. 88 ; also Trans. Eoy. Geol. Soc. Cornwall, vol. ix (1875) p. 181. 



