348 [b.n.p.0. 



this rock) we have a hydrous silicate of aluminium and sodium. 

 When the water of hydration is omitted from the analysis the 

 silica amounts to 52 per cent. The natrolite occasionally 

 passes into analcime, which is a more acid, hydrous silicate of 

 aluminium and sodium. In these zeolites the aluminium and 

 sodium silicates of the felspar have a secondary representation, 

 and the calcium silicate of that primary mineral has its second- 

 ary counterpart in the calcite and the free silica of the 

 chalcedony. 



8. From this point of view it is evident that we cannot 

 draw the line very finely between the terms primary and second- 

 ary, if we regard the former as applied to minerals of igneous 

 origin, and the latter as applied to minerals of aqueous (hydro- 

 thermal) origin. In the formation of such rocks, and the 

 mineral contents of their veins, there is an unbroken sequence 

 of action from igneous to hydro-thermal, and it is possible that 

 a mineral might be formed by a process which is neither a 

 purely igneous one nor purely an aqueous action. We see this 

 in the case of hullite, which, from one point of view, has been 

 described as a basic volcanic glass of igneous origin, and from 

 another as a hydrous green earth of aqueous origin. 



There is a certain amount of truth in each aspect, and the 

 truth in both cannot be reconciled by regarding the hullite, on 

 the one hand, as a volcanic glass altered to a green earth 

 material, or, on the other, as a member of the green earth family 

 derived by solution and decomposition from the primary 

 minerals in the rock. It is more likely that from the beginning 

 the hullite was by nature both a volcanic glass and a green 

 earth. Mineralogists do not seem to have made a clear enough 

 distinction between the two great sources of 'secondary' minerals 

 in lavas — 1st, deposition from the residual waters of the 

 molten magma, and 2nd. deposition from percolating waters in 

 the weathering rock. The action in both cases is an aqueous 

 process, and, therefore, there is a great resemblance, often an 

 identity, between the minerals formed in each way ; but there 

 is a difference in the time of formation and the quantities 

 formed. The first action, at a high temperature, is rapid ; the 



