production of the Prismatic Structure of Basalt. 221 



formed. The facility with which certain varieties of basalt decom- 

 pose and disintegrate is instanced by Bakewell, who, in his work 

 already quoted (p. 127), states that he had " seen a mound formed 

 of basalt that had been got out of a mine by blasting with gun- 

 powder, and which, he was informed, was once extremely hard and 

 resisted the point of the pick ; but by exposure to the air for 

 thirty years it was converted into a rich mould and covered with 

 a luxuriant crop of vegetables." Similar striking differences in 

 facility of decomposition and disintegration have long been re- 

 marked in lavas. Torrents are to be found in abundance which, 

 though poured forth beyond the reach of history or tradition, still 

 resist decomposition, and scarcely permit even a lichen to grow 

 upon their austere and rigid surface, while other examples are 

 abundant of lavas largely decomposed into vegetable earth within 

 a few generations of men ; these facts tend to account for the pre- 

 sence of compact columnar basalt, and basaltic columns decom- 

 posed into onion-stone, existing in close proximity to each other. 

 The notion of production from spheroids is traceable to the 

 paper of Mr. Gregory Watt, in the Phil. Trans, for 1804, entitled 

 " Observations on Basalt," &c. Partly from the backward 

 state of physical and chemical science as applied to geology at 

 that period, this paper at once acquired an exaggerated reputa- 

 tion and currency, and has continued almost unchallenged for 

 seventy years. So far, however, as relates to the production of 

 prismatic basalt in the way assigned by Mr. Watt, it is impos- 

 sible to avoid stating that his opinions rest upon a bad or imper- 

 fect experiment inaccurately reasoned upon and falsely applied. 

 He fused in a reverberatory furnace (whether an iron-foundry 

 ' ' melting-furnace " or a " puddling-furnace " is not clear) some 

 six or seven hundredweight of " Rowley-rag " basalt ; and 

 finding that after slow cooling some parts of the mass consisted 

 of irregular spheroidal lumps of various sizes separated from 

 each other by more or less flattened surfaces, which were red or 

 rusty in colour, he concludes that by fusing and slow cooling this 

 concretionary structure had resulted ; he also infers that a like 

 structure upon a much vaster scale must have been developed 

 in all prismatic basalt, the prismatic structure being produced 

 by the squeezing together of these concretionary lumps into the 

 prismatic form. Now the immediate result of Mr. Watt's ex- 

 periment was not fused basalt alone, but that material hetero- 

 geneously mixed up with the fused brick of his furnace (roof as 

 well as sole), and most probably also with the silicates and oxides 

 of iron adherent to it from former use, and more or less of the 

 ashes of the fuel ; hot sand moreover was employed to cover 

 over the surface of the liquefied mass, so as to cause it to cool 

 more slowly. All these conditions sufficiently show that the 



