350 Scientific Geology. 



to be opposed by the semi-mechanical character which some of these 

 rocks 'exhibit. But waving this difficulty, it is impossible to con- 

 ceive how the materials of all these rocks could have been held in 

 solution by all the waters on and in the globe ; since the earths that 

 form them are scarcely soluble at all in water. Yet even allowing 

 such a solution possible, by what unheard of chemistry was it, that 

 so many distinct minerals as enter into the composition of these rocks, 

 or occur disseminated in them, should have been crystalized at the 

 same instant ? The supposition is opposed by all that we know of 

 the crystalization of different substances in the laboratory in the same 

 solvent. For they crystalize in succession, not simultaneously. But 

 we know that the melted matter of a furnace, if slowly cooled 

 will separate into different compounds ; and the same result takes 

 place in fused basalts, and in the lava of existing volcanoes. Surely 

 then, it seems to me that nature and art both teach us that analogous 

 cases, of crystalization in the rocks must have resulted from igneous 

 fusion. But on the other hand, the foliated structure of the stratified 

 primary rocks, proves that water must have been concerned in their 

 formation ; and they could never have been in a state of complete fu- 

 sion. I am inclined, therefore, to the theory which supposes that they 

 were originally mechanically deposited from water, like the existing 

 secondary and tertiary rocks, and that they have subsequently been 

 subjected to such a degree of heat as enabled their materials to enter into 

 a crystalline arrangement, without destroying their structure. That 

 the two things are compatible, seems probable from the change of 

 bulk produced on solid bodies by slight changes of temperature, 

 showing a motion among the particles; from the changes of crystal- 

 ization that sometimes takes place in solid glass ; from the columnar 

 structure assumed by certain sandstones when in contact with trap 

 rocks ; and from the experiments of Dr. Macculloch, who " proved 

 that every metal can completely change its crystalline arrangement 

 while solid, and many of them at very low temperatures."* This 

 theory also explains why it is that the primary and transition rocks 

 become less and less crystalline the higher we ascend in the series : 

 for the higher they are, the farther they lie from the source of heat. 

 This theory, however, does not suppose that all cases of crystalline 

 structure in rocks has been the result of fusion : for limestone and 

 quartz rock might have been deposited from aqueous solution. But 



* System of Geology, Vol. 1. p. 190. 



