Geology ; as illustrated by Chemistry and Physics. 47 



However, notwithstanding the perfect mineralogical and 

 chemical similarity, phenomena sometimes present them- 

 selves in basalt, which warn us not unconditionally to infer 

 from this resemblance an entirely similar formation. Berger 

 and Maculloch describe basaltic dikes of a few inches, and 

 even less, in thickness, occurring upon the islands of Anglesey 

 and Barra, and intersecting the rock in all directions. 



Let us imagine a mass of matter in igneous fusion, which, 

 after solidifying, yields a basaltic rock ascending in a fissure 

 by volcanic agency, it must not be forgotten that this solidi- 

 fication would take place the sooner the narrower the fissure 

 was. The flowing or ascent of masses in igneous fusion 

 would therefore cease the sooner, the narrower the fissure ; 

 for the narrower it is the greater is the cooling action of the 

 cold surfaces of the adjoining rocks. The width of a fissure, 

 and the temperature of the ascending melted mass, therefore, 

 determine the possibility of a penetration. If the fissure be 

 too narrow, and the temperature of the mass too low, the 

 penetration is no longer possible.* But there is yet another 

 circumstance to be taken into consideration. The more 

 rapid the solidification, the less is it possible for a melted 

 mass to assume a crystalline form. If it takes place too 

 rapidly, and this would be the case in proportion to the nar- 

 rowness of the fissure, then only scoriaceous masses are 

 formed. Therefore ; even if the penetration can be conceived 

 possible, still, if the circumstances are of such a nature that 

 a slow solidification cannot take place, and the mass filling 

 the fissures also appears as a true basalt, then it is necessary 

 to be very cautious in assuming the crystalline basalt to have 

 been formed by igneous agency. It must be left to the Eng- 

 lish geologists to ascertain, by an accurate revision of the 

 narrow fissures and veins filled with basalt, how far the geo- 

 logical relations existing there can be reconciled with the 

 laws of the crystalline solidification of melted masses. 



Before attempting to include the basalt of all localities 

 among the products of melted masses, endeavour should 

 have been made to remove these and other difficulties. But 



Lehrb. der Chemischen u. Physik. Geologie, vol. ii., p. 740, p.t seq. 



