262 J. D. Dana on Pseudomorphism. 



sary magnesia ; and, moreover, exposure to such vapours 

 would have desti'oyed the form of the crystal. 



This view is further corroborated by the occurrence of si- 

 liceous pseudomorphs along with those of steatite. The stea- 

 titic spinels, also, of the same localities, afford other proof, as 

 the change in these crystals has consisted in a substratum of 

 silica for alumina — thus making a silicate of magnesia out of 

 what was before an aluminate of magnesia — a change that 

 requii-es water and heat for the solution of the silica, and 

 heat no less for effecting the transfer of elements. 



The fact that the majority of these pseudomorphs contain 

 water, is other proof that the heat producing the change was 

 not a dry heat, but was diffused through water. Dr Blum 

 does not seem to recognise the fact of the hydi'ous character 

 of the steatitic pseudomorphs in his work ; yet analyses have 

 shewn, that, in very many instances, the steatite is a hydrous 

 mineral, and not true steatite. Serpentine and chlorite, also, 

 contain a large per-centage of water.* 



* Tlie agency of hot waters and vapours in producing changes in 

 rocks, though recognised bj many geologists, does not seem to have re- 

 ceived sufficient attention. When we consider the number of liot springs 

 on the surface of the earth, in regions of modern volcanic action, and in 

 others not of this nature — when we remember the many eruptions of hot 

 water, even from subaerial volcanoes — and when, further, we have before 

 our ej'es the wide-spread effect of volcanic action beneath the sea, can 

 we refuse to the agency of heat, thus conveyed by vapours and flowing 

 waters, a lar^e jMrt, at least, of the various metamorphic changes mani- 

 fest on the surface of the earth, especially if we take into view the con- 

 dition of a vast volcanic region in full submarine action, its floods of 

 melted rocks, its opened fissures, for a long period in action, its foun- 

 tains of boiling waters and rising vapours .'' The whole surface of the 

 earth evijices that this submarine action, under an ocean's pressure, has 

 been almost co-extensive with it. The vast number of dikes that tra- 

 verse the rocks suggest the same fact. Where are the effects of this 

 heat, if not in these pseudomorphic and metamorphic changes ? Professor 

 Keilhau attempts to account for the crystallization of the dolomites, by 

 the hypothesis of slow molecular changes taking place at the ordinary 

 temperature. But why refuse the aid here proffered .'' The fact that 

 one part of a sedimentary limestone is crystallized, while another is no+, 

 is fully explained by this view; while his theory serves only to increase 



