478 
ON CHEMICAL GEOLOGY. 
most extraordinary changes may be produced by superadding pressure. By this means, 
the application of a gentle heat and moderate pressure for several months, ordinary clay 
slates may be converted into crystalline rocks, having precisely the appearance of gneiss. 
Limestones, from similar causes or by the infiltration of water charged v#th carbonic 
acid, are also converted into crystalline rocks; or they may undergo conversion into do¬ 
lomites in consequence of the access of water holding magnesian salts in solution. 
The grand cause of metamorphism is not what has been termed “ hydrothermic action.” 
In order to produce imitations of metamorphic rocks, it is only necessary to employ a 
gentle heat, not sufficiently great to cause softening; the pressure applied must be con¬ 
stant, and oxidation prevented. It may be, therefore, confidently asserted that the 
structure of the oldest rocks did not arise from cooling from a state of fusion, but that 
they were metamorphosed whilst still solid. 
At the conclusion the Chairman (Dr. Williamson) moved a vote of thanks to the 
lecturer. 
Sir Roderick Murchison congratulated geologists upon the able manner in which 
Mr. Forbes had grappled with some most difficult questions. Numerous geological phe¬ 
nomena of the highest interest could only be explained satisfactorily by an appeal to 
chemical investigation. 
Professor M ‘Donald acknowledged himself an adherent of the Neptunian system; 
and asserted that if the cavities in the quartz found in granite be examined with suffi¬ 
cient care, they would be found to be of definite shape and lined with crystals, quite 
different from the spaces existing in volcanic lavas, and that in addition they very often 
contain explosive fluids. 
Professor Morris held the opinion that zeolites had been crystallized from water. 
These minerals never occurred in modern lavas, but were generally found in the older 
rocks. The question of metamorphism was still little understood, and required eluci¬ 
dation from the chemist. The speaker considered that minerals would ultimately come 
to be regarded as of nearly the same importance as fossils in determining the age of 
rocks. 
Dr. Hugo Muller announced his concurrence with many of the views maintained by 
Mr. Forbes, but differed from that gentleman in his opinion as to the origin of the quartz 
in some of the granitic rocks. Mr. Forbes had compared the separation of quartz crystals 
to that of graphite from pig-iron, (a beautiful specimen of which was upon the table), 
but the analogy appeared to be only imperfect, inasmuch as in the case of the graphite 
this being very infusible separates in the crystalline state as soon as its affinity for the 
iron diminishes ; whereas in granite it is found that the felspar, although more fusible, 
crystallizes first. The experiments of Mr. Sorby had shown that the highest tempera¬ 
ture at which the cavities in quartz and other minerals are formed is about 400° C. or 
about the melting-point of lead. This, however, is hardly a temperature to be associated 
with igneous action, being such as any of the sedimentary rocks may have been subjected 
to at different times. The trachytes of Ponza and Palmarola contain crystallized quartz, 
the water cavities of which, according to Mr. Sorby, were formed at a temperature of 
about 360° C. This fact the speaker regarded as proof that this quartz is a secondary 
product not formed at the time of the eruption of the lava, since it is impossible to be¬ 
lieve that quartz could remain fluid at the temperature of melting lead. The fact that 
quartz and zeolites have been taken from modern lava is not more conclusive, for the 
opinion formed by the speaker, after a personal inspection of the Monte Somma formation, 
is that fragments of rock, crystallized minerals, and other de'bris of the ancient volcano 
are thrown up mechanically mixed with the lava which has been ejected in modern 
times. 
Dr. B. H. Paul maintained that some broad principles should be laid down upon 
which chemists might proceed in their investigations in connection with geology. 
Schistose rocks underlie, or did formerly, other sedimentary strata; and it was a matter 
of importance to ascertain what chemical causes were involved in producing the difference 
between them. 
Dr. A. W. Williamson had listened to the lecturer with great pleasure, and agreed 
with him in most of his arguments. There was, however, one point in his ‘ Chapter of 
Genesis ’ which seemed somewhat unintelligible. It had been stated that the consti¬ 
tuents of the earth’s original atmosphere at the time of their formation would arrange 
