Notices respecti7ig New Books. 299 



ference between ordinary volcanic products and those commonly 

 called trap. This pressure is no doubt often due to the presence of 

 a deep sea over the point of eruption, but the hydrostatic pressure 

 of an ascending column of lava is a still more powerful agent. 

 The lava in a volcanic chimney may be compared to the scum 

 and slag in an iron furnace, while trappean rocks represent the 

 heavy and fluid metal which lies beneath it. The pressure of a 

 column of fluid lava 10 or 20,000 feet high would inject the denser 

 matter below into every accessible crevice, horizontally as well as 

 vertically, and when exposed in after ages by denudation, it would 

 exhibit those dykes and tabular masses of basaltic rocks which Dr. 

 Daubeny refers more exclusively to submarine eruption. 



Our author attributes the columnar and jointed structure of 

 basalt to its aggregation, in the act of cooling, into spheroidal masses, 

 which by their mutual pressure acquire a subhexagonal form. It 

 appears to us, however, that contraction, and not pressure, is the 

 " appropriate idea " to be applied to the prismatic structure of 

 basalt. Dr. Daubeny, indeed, maintains that the columns often 

 approximate so closely that no contraction can have taken place. 

 But the fact of the rock separating into columns proves tliat a cer- 

 tain interval exists between them, and the smallness of this interval 

 merely shows that the contraction was small in amount. From the 

 moment that the basalt begins to sohdify, the cooling process must 

 be accompanied by contraction, and although the rock may then 

 have a tendency to assume a spheroidal or concretionary structure, 

 we do not see how such concretions can exert a mutual pressure at 

 the same time that they are separating from each other by con- 

 traction. Moreover, the spheroidal structure is often absent, and the 

 basaltic prisms then present a homogeneous texture through their 

 whole length. The subhexagonal structure would therefore appear to 

 be the first or normal condition which a tabular mass of basalt tends 

 to assume, and the spheroidal arrangement to be the offspring and 

 not the parent of that structure. 



Tliis volume is appropriately concluded by a chapter on the final 

 causes of volcanos, in which those who have exclusively regarded 

 these igneous o23erations in the light of destructive agents, will be 

 gratified by some sound and philosophical views as to the benefits 

 which they confer on the organic creation. . We will quote the fol- 

 lowing as an example : — 



" Potass, soda, certain earthy phosphates, lime, magnesia, must 

 be present wherever a healthy vegetation proceeds. Now some of 

 these bodies are naturally insoluble in water, whilst others are dis- 

 solved with such readiness, that any conceivable supply of them, in 

 their isolated condition, would be si)ecdily carried off^ and find its 

 way into the ocean. The first, therefore, must be rendered more 

 soluble, the latter less so, than they arc by themselves. Now the 

 manner in which nature has availed herself of the instrumentality of 

 volcanos to effect both these opposite purposes is equally beautiful 

 and sinijjlc. 



" She has in the first place brought to the surface, in the form of 



