§ 51. HERTFORDSHIRE PUDDING-STONE. 101 



melted by water : and there was no appearance of the 

 smooth glazed surface, which invariably attends the action 

 of an alkali on a siliceous substance. Some of the earthen- 

 ware vessels were partially eaten through ; but on the 

 uppermost arch, where the heat was only a full red, a 

 curious phenomenon appeared ; the articles there had 

 received, exterior to their own glazing, and loosely incrusting 

 it, a complete frosted siliceous coating, having the appear- 

 ance of a candied surface. This was evidently a precipi- 

 tation from the vapour ; in fact, a hoar-frost of silex. 

 There was from half an ounce to an ounce on each vessel, 

 and altogether several pounds were thus precipitated ; but 

 by far the greater part of the vapourized mineral was, as 

 might be expected, carried away by the current, and dissi- 

 pated in the air. This powerful action was apparently 

 entirely due to the presence of water ; there being at all 

 times the same quantity of alkali present in the fuel, whatever 

 that might have amounted to, without producing such an 

 effect. The experiment is conclusive as to the solvent 

 power of water at a very high temperature on silica, and 

 siliceous rocks ; for the action cannot be attributable to 

 the alkalies, because, under precisely similar conditions, the 

 experiment failed when there was no water ; and besides, 

 each pound of alkali would have had to dissolve forty 

 pounds of silica. Mr. Jeffery has in fact performed, on a 

 small scale, the same operation which is incessantly going 

 on in the volcanic regions of Iceland and New Zealand, 

 and established the potency of heated water and vapour 

 to effect the solution of silex.* 



51. Hertfordshire pudding-stone. — We have before 

 us numerous examples of conglomerates formed by car- 

 bonate of lime ; in other words, aggregations of pebbles, 



* See an account of Mr. Jeffery's experiments in Reports of the 

 British Association of Science, for 1840. 



