360 Notices of Memoirs. 



derived solely from lithological characters, as the recent and un- 

 changed forms of life may very readily be compared with the fossil 

 forms, and their structure and composition is of a more definite 

 character. 



The work is divided into seven articles. Some of the principal 

 theories of metamorphism are first pointed out and discussed ; the 

 alteration by heat of gneiss and other rocks exclusively termed 

 metamorphic is objected to. When, as is sometimes the case, the 

 stratification is apparent, and the character uniform, of beds hun- 

 dreds of metres in thickness, the evidence seems to the author at 

 once opposed to the igneous theory. A very elevated temperature 

 would in places have destroyed every trace of stratification, while 

 the change would be gradual, fading away from the point of contact 

 with the igneous rock. To account for the uniform character we 

 should have to suppose that the mass was entirely surrounded by 

 molten material. The author points out the slight facility with 

 which the majority of rocks conduct heat. 



Several instances are quoted where rocks, undoubtedly of sedi- 

 mentary origin, have originally been taken for igneous and eruptive 

 rocks. It is also mentioned that volcanic lavas may, by atmospheric 

 influence, be decomposed into clays, and undergo the same trans- 

 mutations as the aqueous rocks. 



The petrifaction of organic remains teaches us that the particles 

 composing rocks do not remain inert ; they indicate molecular changes. 



Some space is devoted to the changes that are evidenced by mol- 

 lusca. Many instances of replacement, both entire and partial, are 

 noticed. It is evident in general that this substitution must have 

 proceeded atom by atom. 



In many instances the shell has been more or less removed with- 

 out being replaced ; but the author remarks that, during his long 

 researches, he has in no instance observed the shell entirely dissolved 

 in such a manner as to allow the shock to be heard of the nucleus 

 inside the cavity. Thus occasionally new matter may fill these 

 cavities, and so replace the shell in its general form, without re- 

 placing the intimate structure. 



The most metamorphosed rocks have undergone the greatest amount 

 of chemical action, or a repetition of metamorphic actions. In the 

 earlier stages the changes were in a great measure confined to the 

 organic remains, and these seem to have suffered much, so that in 

 the more altered rocks they are greatly changed, or even destroyed. 



Crystallisation appears to be the last stage of metamorphism, — it 

 often destroys the organic remains. 



Professor Montagna, however, announces his discovery, in granite, 

 of vegetable remains, consisting of portions of Lepidodendron iden- 

 tical with species found in the Coal-measures ; also of Lepidodendron 

 sexangulare in a compact serpentine, and in certain porphyries. 

 Vegetable remains are also cited as occurring in syenite. About 

 seven-tenths of the numerous specimens of granite and granitic 

 rocks examined by him prove their aqueous and stratified origin, by 

 yielding plant-remains, which are more evident the finer the grain 



