192 



ME. JUKES-BEOWNE AND PEOE. HAEEISON 



minute amounts, which in part are doubtless derived from alteration 

 of colloid silica, in any of the Oceanic Deposits. The clays of the 

 South American littoral moreover contain considerable proportions of 

 potash, not unfrequently amounting to over 2 per cent., and this 

 is also the case, although to a lesser extent, with the clays derived 

 from the coral-rocks and with the Scotland Beds, whilst the argil- 

 laceous earths contain but minute amounts of this alkali. 



(e) The Layers of Volcanic Sands or Ashes and Mudstones. — The 

 following table gives compositions of three of the interbedded layers 

 which consist so largely of volcanic material ; that of a sample of the 

 volcanic sand or dust from the Souffriere of St. Vincent, which fell 

 upon Barbados in 1812, and is locally known as May Dust,^ being 

 included for comparison. The layers from which the samples 

 analysed were taken are, 1st, a bed varying from 7 to 10 inches 

 in thickness, exposed in Consefc Gully (see p. 205) ; this is of a 

 dark brown colour, containing, as do several of the thinner beds of 

 volcanic material, a considerable quantity of petroleum (derived 

 probably from the lower Scotland Beds) ; 2nd, the bed of 

 pumiceous sand, near Ohimborazo, at the top of Melvin's Hill (see 

 pp. 180, 208) ; and 3rd, the bed of grey mudstone from the summit of 

 Mount HiUaby (see pp.' 180, 209). 



Conset Melvin's Mount 



Gully Hill (rather HiUaby (fine May Dust 



(coarse). coarse). mudstone). (sandy). 



Loss on ignition 11-50 -41 2-23 281 



Quartz -65 '12 '04 -26 



Colloid silica 18-46 28-50 19-34 1247 



Combined silica 47-31 38-31 44-22 41-52 



Alumina 9-39 14-59 16-55 21-85 



Iron peroxide 1-55 1*07 trace 1-81 



Iron protoxide 3*72 2-15 5*43 6-33 



Manganese oxide 3-44 ... '09 '03 



Manganese peroxide 4*24 



Lime 2-10 223 7'59 7-16 



Magnesia -90 1-25 1-57 1*99 



Potash 1-25 1-94 -36 104 



Soda trace 5-21 2-58 2-99 



100-27 100-02 100-00 100-26 



In these layers of volcanic materials soluble or colloid silica is 

 always present in considerable quantity, partly in the form of 

 siliceous organisms, but largely as a cementing material. It is also 

 present, but in much smaller quantity, in the so-called May Dust, 

 in which it occurs in the form of siliceous organisms of freshwater 

 origin, and as finely divided silica. The general resemblance in 

 composition between the sandy dust known to have come from 

 St. Vincent, and the layers occurring in the Oceanic Series, when 

 the increased amount of the colloid silica in the bedded layers is 

 allowed for, points to a similar origin, that is, from a lava containing 

 but little free silica. Two of the samples (those from Conset Gully and 



^ The ' May Lust ' is said to have been collected at the time of falling. 



