Marshall. — On Tridymitc-TracJiyte of LyttcUon. 381 



Although fully recognising the danger of theorising upon 

 any subject when the data are insufficient, I cannot refrain 

 from offering a suggestion as to the cause of silica crystallizing 

 in the form of tridymite. The available data to reason upon 

 are — 



1. The invariable presence of tridymite in cavities ; 



2. Its frequent radial structure ; 



3. Its non-inclusion in other crystals. 



All these point to the conclusion that the mineral was 

 deposited subsequently to all the other minerals of the rock, 

 and from a solution in some solvent. Silica if uncombined is 

 well known to be insoluble in pure water at atmospheric pres- 

 sure ; but there seems to be every reason to suppose that it is 

 soluble in water above its critical temperature and under con- 

 siderable pressure. The experiments of Daubree with water in 

 glass tubes confirm this, and the theory to account for the 

 fact that quartz is the last mineral to crystallize in granite 

 shows that it is generally believed that silica can be dissolved 

 in water of a sufficiently-high temperature and under great 

 pressure. In lava, prior to its extrusion, the water present 

 would probably be capable of dissolving silica, but on the 

 eruption of the lava the pressure would be suddenly relieved 

 and the temperature quickly lowered, and consequently the 

 silica would be precipitated. Owing to this rapid precipitation 

 the silica molecules might be unable to arrange themselves in 

 their ordinary arrangement, or the molecular forces, owing to 

 direct solution or the subjection to heat, might well be 

 different from the ordinary molecular forces, and would tend 

 to form a different geometrical solid from that formed under 

 the normal conditions. 



That the molecular forces under certain conditions are 

 liable to variation is w^ell shown by the production of dimor- 

 phous forms in Ca CO^, native sulphur, and other minerals ; 

 and there seems to be no physical reason why variation 

 should not exist in the case of others when the conditions 

 accompanying crystallization also vary. 



The extreme minuteness of the individual crystals would 

 seem to indicate rapid precipitation, while inclusions of steam 

 or other fluids would easily find their escape through the 

 divisional planes between the different crystal plates. 



It does not seem probable that higli temperature alone, 

 or sudden cooling, determines tlie system in which silica crys- 

 tallizes, since in rhyolites and other lavas in which these con- 

 ditions obtained the silica crystallized in the form of quartz. 

 In the case of holocrystalline rocks, on the other hand, the 

 cooling was extremely slow ; and in the case of older lavas the 

 molecules may have had time to readjust themselves in accord- 

 ance with the normal forces after extrusion. In recent lavas 



