848 
[B.N.F.C. 
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 1 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 — <ist, 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 
