22 Victoria as a Field for Geologists. 



same Avill combine with the alkali present, and the contained 

 silica will be thrown down in the form of a jelly. But when 

 the acid is added in another way, the silica is not precipi- 

 tated. If, then, we take a small hoop of gutta percha, ha\^ng 

 a bottom formed of parchment paper, and place the sa.me 

 floating in waiter, and if, whilst so floating, we pour into it 

 some of the last mentioned liquid, we shall have the following 

 result. The acid and the alkali will pass through the paper, 

 as will also a small portion of the silica, but another portion 

 of the latter will remain and be suspended in the water. In 

 man}^ instances as much as six per cent, remained so dissolved, 

 and that without there being the slightest particle of base to 

 thus keep it in solution. This is the process of dyalisis, and 

 when, as is stated in the lecture referred to, it is found that 

 all the conditions of the artificial process are likely to exist 

 in a state of nature, there is reason to suppose that the solu- 

 tion and redeposition of vast masses of siliceous matter, by 

 aqueous agenc}^ alone, are by no means so difiicult as was 

 once imagined. 



There is another way, but as I have never heard the same 

 put forward by any writer on geology, you will please to 

 receive the same with caution, and consider it rather as a 

 query put to elicit truth, than as a dogmatic assertion ex- 

 planatory of a difiiculty. 



May not both aqueous and igneous agenc}^ have come into 

 operation ? May not the silica, mixed with lime, alumina, 

 and potass, as fluxes, have first, as a fluid mass, filled the 

 fissures ; and may not the same have been subsequently 

 metamorphosed and altered to its present form by the 

 removal of the flux in question ? 



As bearing upon this portion of the subject, permit me 

 again to draw an illustration from Dr. Percy's lectures. It 

 is the account of an experiment descriptive of the artificial 

 production of staurolite. A porcelain tube was taken, and 

 after being filled with alternate layers of alumina and silica 

 (as in sketch), the whole was placed in a furnace and main- 

 tained at a red heat, whilst a continuous stream of fiuoride 

 of silicon was made to pass through it. On the tube being 

 broken up, it was found to contain, not separate layers of 

 the two elements, but a compound formed of both silicate of 

 alumina or staurolite, as required. The rationale is as fol- 

 lows : The fiuoride first deposits its silica in the alumina, 

 taking up alumina instead. It is then fluoride of alumina, 

 and passing on through the next layer it is changed back to 



