241 
THE GEOLOGIST. 
made known the results of Ids rescairlies ou the aUcratiuii of com- 
bustibles — such as lignite, coal, anthracite, &c. — by the action of 
crujjtive or phrtonic rocks. Wc arc happy to say that this distin- 
guished geologist perseveres in the study of metamorphism, upon the 
phenomena of Avhich he has already thrown so much light. In one of 
his recent memoirs, he has reviewed the action produced by lava and 
traj^-rocks on the different limestone strata with which they have come 
in contact. 
In the case of lava, either ancient or modern, the metamori^hism 
2)roduced seems to be entii-ely the result of great heat. By contact with 
lava, limestone has become crystalline, and has taken a saccharoid ap- 
pearance. A great number of minerals are also formed in limestone 
strata by the metamorpliic action of lava. M. Delesse has remarked, 
in particular, garnets, idocrase, epidote, pyroxene, and mica, all of which 
are observed at Vesmdus in the limestone of La Somma. Many 
stratified rocks M'ith which lava has come in contact have been split or 
fissured, and have taken a reddish tint, which they had not before. 
This coloration, which is pretty general, appears to us to lie due to the 
action of the atmosiihere upon the protoxide of ii'on contained in the 
rocks at the time of their contact. This protoxide generally produces 
green or black-coloured silicates in volcanic and other plutonic rocks ; 
but, if these are violently heated in the air, a certain quantity of the 
protoxides contained in their silicates of iron is transformed into 
peroxide, which is red. 
Wlien the heat })roducetl during the eruption of the lava has been 
very great, the rocky strata upon which it has acted have become 
jiartially or wholly vitrified. It has lieen remarlced, however, that 
rocks, sandstones, &c., are not changed into transparent c^uurtz 
by the metamorpliic action of lava ; on the contrary, their silica 
combines with the different oxides furnished by the latter, and thus 
are 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, eupliotide, diorite, and ordinary 
trap — is observed to have been more marked on calcareous strata in 
those parts where the erupted mass has been greatest. The change 
undergone T>y the limcstoue-rock rarely extends, however, to more than 
