I.] THEORY OF IGNEOUS ROCKS AND VOLCANOES. 9 



which I have here brought forward. In a letter to Sir Charles 

 Lyell, dated February 20, 1836 (Proceedings Geol. Soc., Lon- 

 don, Yol. XL p. 548), he maintains that with the accumulation 

 of sediment the isothermal lines in the earth's crust must 

 rise, so that strata buried deep enough will be crystallized and 

 metamorphosed, and eventually be raised, with their included 

 water, to the melting-point. This will give rise to evolutions 

 of gases and vapors, earthquakes, volcanic explosions, etc., all 

 of which results must, according to known laws, follow from 

 the fact of a high central temperature ; while from the me- 

 chanical subversion of the equilibrium of pressure, following 

 upon the transfer of sediments, while the yielding surface 

 reposes upon a mass of matter partly liquid and partly solid, 

 we may explain the phenomena of elevation and subsidence. 

 Such is a summary of the views put forward more than twenty 

 years since by this eminent philosopher, which, although they 

 have passed almost unnoticed by geologists, seem to me to 

 furnish a simple and comprehensive explanation of several 

 of the most difficult problems of chemical and dynamical 

 geology. 



To sum up in a few words the views here advanced. "We 

 conceive that the earth's solid crust of anhydrous and primitive 

 igneous rock is everywhere deeply concealed beneath its own 

 ruins, which form a great mass of sedimentary strata, per- 

 meated by water. As heat from beneath invades these sedi- 

 ments, it produces in -them that change which constitutes 

 normal metamorphism. These rocks, at a sufficient depth, 

 are necessarily in a state of igneo-aqueous fusion, and in the 

 event of fracture of the overlying strata, may rise among them, 

 taking the form of eruptive rocks. Where the nature of the 

 sediments is such as to generate great amounts of elastic fluids 

 by their fusion, earthquakes and volcanic eruptions may result, 

 and these, other things being equal, will be most likely to 

 occur under the more recent formations.* 



[NOTE to page 2. I have since pointed out that the evidences of a 

 similar process are still to be seen in the granites and crystalline schists 



* See further in this connection Essays VI. and VII. 



