296 Professor W. J. Sollas — Order of Consolidation 



occupies a position in the order of consolidation corresponding with 

 that of the order of fusion. Similarly labradorite, with a fusion- 

 point between enstatite and augite, usually follows enstatite and 

 precedes augite in order of consolidation. That it is as a rule 

 extricated from the magma before augite, is illustrated by the 

 frequent occurrence of ophitic structure in dolerite. Albite fuses at 

 a lower temperature than augite, and usually crystallizes after it. 



It would be natural to suppose that the species of felspar inter- 

 mediate with labradorite and albite in composition would be 

 also intermediate as regards their fusion-points, and if so, the 

 irregularities which distinguish the succession of plagioclase and 

 pyroxene might find an explanation in the fact that the fusion- 

 point of augite stands almost midway between that of albite and 

 labradorite. 



The potash felspars have a lower fusion-point than albite, and, 

 as is well known, generally succeed it in order of consolidation. 



The order of fusion of orthoclase, sodalite, and nepheline is 

 repeated in the order of consolidation of these minerals in nepheline 

 basanite ; but an anomaly is presented in the case of some examples 

 of Eeleolite syenite, in which nepheline and leucite precede 

 orthoclase in order of consolidation. To this allusion will be made 

 later. The most important exception to the rule that the order 

 of fusion corresponds to the order of consolidation is afforded by 

 quartz, which with a fusion-point of 1425° crystallizes later than 

 orthoclase with a fusion-point of only 1164:°-1168°. An explanation, 

 however, may be found for this, and if admitted may throw light 

 on some other inversions of the rule. The statement of Zirkel will, 

 I think, be generally conceded. "A mineral may crystallise out 

 from the molten silicate-magma at the most various temperatures — 

 naturally, however, never ahove its own fusion point." ^ 



The fact to be explained therefore in the case before us is the 

 consolidation of quartz at a temperature below the fusion-point of 

 orthoclase, i.e. at least 260° below its own fusion-point. In the report 

 of the meeting of the British Association for 1882 will be found an 

 account of some experiments made by Mr. Hunter for a committee of 

 the Association (p. 239). Some chemically pure silica, "prepared by 

 precipitation from sodium silicate with hydrochloric acid, evaporation 

 to dryness, thorough washing, and subsequent ignition, was placed in 

 a sealed iron-tube and heated with water to 300° C. for two days. 

 At the conclusion of the experiment the impalpable powder of silica 

 was found to have caked together into a white opaque granular 

 mass." It consisted of anhydrous silica, as was proved by chemical 

 investigation. The results of an examination which I made under 

 the microscope were as follows : — " It was found to have passed 

 into a state of glass. The glass is transparent and colourless and 

 hard enough to scratch ordinary window glass ; it is, however, filled 

 with innumerable oval and tubular cavities, so as to resemble pumice, 

 and it is to them that it owes its whiteness and opacity when seen 

 by the unassisted eye." 



1 Zirkel : "Lehrbuch der Petrographie," 1893, vol. i, p. 728. 



