244 



THE GEOLOGIST. 



made known tlie re.sults of liis researciies on the alteration of com- 

 bustibles — sueli as lignite, coal, antliracite, &c. — by the action of 

 eruptive or jDlutonic rocks. We are happy to say that this distm- 

 guished geologist perseveres in the study of metamorpliism, upon the 

 phenomena of which he has abeady throT^Ti so much light. In one of 

 his recent memoirs, he has reviewed the action produced by lava and 

 trap-rocks on the different limestone strata with wliich they have come 

 in contact. 



In the case of lava, either ancient or modern, the metamorpliism 

 produced seems to be entirely the result of great heat. By contact with 

 lava, hmestone has become crystalline, and has taken a saccharoid ap- 

 23earance. A great number of minerals are also formed in limestone 

 strata by the metamorphic action of lava. M. Delesse has remarked, 

 in particular, garnets, idocrase, epidote, pyroxene, and mica, all of which 

 are observed at Vesuvius in the limestone of La Somma. Many 

 stratified rocks with which lava has come in contact have been split or 

 fissured, and ha^^e taken a reddish tint, which they had not before. 

 This coloration, wliich is pretty general, appears to us to be due to the 

 action of the atmosphere upon the protoxide of u^on contained in the 

 rocks at the time of their contact. This protoxide generally jiroduces 

 green or black- coloiu-ed silicates in volcanic and other plutonic rocks ; 

 but, if these are violently heated in the au', a certain quantity of the 

 protoxides contained in their silicates of iron is transformed into 

 peroxide, which is red. 



When the heat produced during the eruj)tion of the lava has been 

 very great, the rocky strata upon which it has acted have become 

 partially or wholly ^dtrified. It has been remarked, however, that 

 rocks, sandstones, &c., are not changed into transparent quartz 

 by the metamorphic action of lava ; on the contrary, their silica 

 combines with the different oxides furnished by the latter, and thus 

 arc produced certain silicates which have a vitreous appearance, 

 thereby resembling quartz of a cellular structure. 



Tlie metamorphic action of trap-rocks — among which M. Delesse 

 mentions basalt, dolerite, hyperite, euphotide, diorite, and ordinary 

 trap — is observed to have lieen more marked on calcareous strata in 

 those parts where the erupted mass has been greatest. The change 

 \uidorgonr by the limcstune-roek rarely extends, hoAvever, to more than 



