IxXX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



effected, — in fact, that a potter's glaze, as Sir James pointed out, was 

 made, or in other words, that a siUcate of soda was produced which 

 cemented the grains of sand, forming a sandstone. If we follow out 

 this view, and bear in mind that every detrital deposit in the sea, — 

 mud, silt and sand, — is well saturated with sea water, and conse- 

 quently has much chloride of sodium disseminated amid silicates and 

 silica, often in a minute state of division, we shall see that if suffi- 

 cient heat he applied to such detrital sheets of matter so saturated, 

 we have the conditions for the formation of the silicate of soda. The 

 needful amount of heat may either be applied by the intrusion of a 

 mass of molten rock, such as granite, or by sinking the mass of 

 matter to great depths, and beneath coverings of more recent accu- 

 mulations. 



No doubt, when we regard the subject in this light, there are very 

 many other things to be considered than the mere application of 

 heat, and the presence of silica and chloride of sodium . Many of the 

 complications wdiich would arise will readily suggest themselves to 

 you. We merely desire to call your attention to the production by 

 such means of silicate of soda, among other changes and modifications. 

 Certain of the silicates of soda which might be so formed may be 

 soluble, so that during the circulation of moisture amid the fissures, 

 joints, cleavages, beds and pores of rocks, they may be removed, while 

 others may not be so, under the conditions, but remain cementing the 

 portions of detrital matter together, turning parts and even whole 

 sheets of friable deposits into hard rocks. 



We are far from supposing that silica may not be, and is not ob- 

 tained in solution in various other ways, and be thus transported 

 from one place to another. Other means are sufficiently obvious ; but 

 it seemed not undesirable to recall your attention to the experiments 

 of one, who laboured so earnestly in the promotion of our science, at a 

 time when experimental investigations were less appreciated than 

 many an unsupported and often wild assumption. It may scarcely 

 be necessary to remind you, that when such a solution as we have 

 noticed met with carbonic acid it would be decomposed, and the silica 

 set free, to be borne onwards with the moisture or water, and de- 

 posited, according to conditions. You are fully aware of the decom- 

 position of the silicates of soda or potass of the felspar family by 

 means of carbonic acid, even that in the atmosphere, and the conse- 

 quent state of the silica under such circumstances. 



Although springs show us many solutions which have been effected 

 by means of water traversing rocks, especially when percolating 

 through mineral masses elevated above the level of the sea, thus 

 washing out many a substance from them which became disseminated 

 amid their constituent parts when accumulated beneath the sea, or 

 produced by subsequent conditions, — chloride of sodium very com- 

 monly, * — they do not always give us the conditions of the substances 



* This common presence of chloride of sodium is a very interesting circum- 

 stance. When submarine-formed deposits, moistened with a solution of it, are 

 elevated into the atmosphere, it becomes by degrees washed out of them, so that 

 the longer they have been exposed during the lapse of geological time to this 



