CAUSES OF CEMENTATION. 633 



the increase in volume is 20.82 per cent (p. 375). Supposing that the 

 anorthite by hydration passes into thoinsonite, a zeolite with only a mod- 

 erate amount of water, the increase in volume is 34.65 per cent (p. 376). 

 Supposing that the diopside by carbonation, hydration, and oxidation passes 

 into talc, quartz, magnetite, and calcite, the increase in volume is 27.88 

 per cent (see sahlite, p. 391). Supposing that the hypersthene by hydration 

 and oxidation passes into talc, magnetite, and quartz, the increase in volume 

 is 12.84 per cent (p. 385). Supposing that the olivine by hydration and 

 oxidation passes into serpentine, magnetite, and quartz, the increase in 

 volume is 15.19 per cent (p. 388). Since the percentages of the other 

 constituents are small, they will be supposed to remain unaltered. In the 

 above table, column (4) is the volume ratio and (5) the volume of resultant 

 products. 



It thus appears that the supposed reactions of hydration, carbonation, 

 and oxidation of the amphibole gabbro result in an increase of volume of 

 25.21 per cent. It is to be noted also that the calculation is made upon a 

 conservative basis. It might be supposed, for instance, that the anorthite, 

 the most abundant mineral in the rock, passed into gismondite, which 

 would increase its volume by 52.76 per cent rather than 34.65 per cent. 

 Also, if the conditions were very favorable for oxidation the iron would 

 pass to hematite and perhaps to limonite. If all these suppositions were 

 made in favor of getting the maximum volume, this would make the volume 

 increase considerably greater than calculated. 



While the scoriaceous upper parts of lava beds may contain a pore 

 space greater than 20 per cent, such a proportion of openings is compara- 

 tively rare for any considerable thickness of lava beds. It is well known 

 that the amygdules of amygdaloids are ordinarily filled with quartz, with 

 the zeolites, with chlorites, with epidotes, serpentine, and other hydrous 

 silicates, and with carbonates; in other words, with the minerals mentioned 

 on pages 621-627 as especially likely to be formed in the belt of cementa- 

 tion. That from the anhydrous silicates, hydrous silicates should form and 

 fill the openings is to be expected. That the bases should unite with carbon 

 dioxide is natural. But what is the source of the quartz! So far as car- 

 bonation of the silicates has taken place — and the presence of plentiful 

 carbonates shows that this is an important reaction — silica, probably mainly 

 in the form of colloidal silicic acid, is liberated. The precipitation of this 



