12 Johns: The Liifirc Felspars of Shap Granite 



case of mag'netite it would appear from the writer's own 

 experiments that its solubility also depends on the acidity of 

 the magma, for he found magnetite to refuse to enter into 

 solution with fused silica at a temperature exceeding i8oo°c. , 

 though it seemed to combine with certain silicates. The 

 presence of magnetite in the consolidated rock suggests that 

 the original magma contained both ferric and ferrous oxides of 

 iron, though only the former is mentioned in the analysis given 

 in the paper quoted from. From experiments upon slags it 

 has been concluded that magnetite never forms unless both 

 oxides are present. If we consider that before it could form 

 from ferric oxide alone, an atom of oxygen would have to be re- 

 moved, the reasonableness of this conclusion becomes apparent. 



When the temperature fell to the critical point corresponding 

 to the separating out of orthoclase from the solution, there 

 would only be in existence those minerals possessing low 

 solubilities. Being a magma, we can also assume that the 

 temperature gradient of the cooling mass was not a steep one. 

 The skeleton frames of the felspars would begin to grow as the 

 embryo crystals formed, and as the particles of orthoclase 

 passed from the fluid into the solid condition heat would be 

 liberated ; from the size ultimately reached by the crystals it 

 becomes evident that the evolution of heat during the solidi- 

 fication of the early orthoclase must have nearly, if not quite, 

 balanced that lost by the magma through its containing walls. 

 But felspar, we know, forms slowly, so our early assumption 

 that the temperature gradient was not falling steeply receives 

 confirmation. 



There is, however, further proof of this from the evidence 

 we have that connection currents were absent within the mass. 

 Rapid cooling would have meant movement of the different 

 particles in the direction in which heat transference was taking 

 place. Now the large felspars could not have reached their 

 abnormal size had the magma been in motion, for it is a 

 common experience with solutions that the more rapid the 

 cooling the smaller are the crystals, and also that motion tends 

 to reduce the size. The reason for this is not far to seek ; the 

 skeleton crystals are very fragile, and if the surrounding 

 mother liquor be in motion they break up and form separate 

 units. Thus far it is very clear ; but these conditions of 

 stillness in the magma and the abnormal growth of the skeleton 

 crystals would be the very ones to render it probable that a 

 portion of the mother liquor would be enclosed in the meshes 



Naturalist, 



