244 SCIENCE PROGRESS 



substance is an integral part of all igneous rock-magmas, nor is 

 the changed fusibility of the minerals peculiar to granite or 

 to plutonic magmas as a class. 1 The lavas poured out from 

 Vesuvius must certainly be regarded as true igneous rocks ; but 

 everybody who has examined these has seen crystals of augite 

 enclosed by those of leucite, though the melting-points of 

 augite and leucite, taken separately, are about 1200° and 1420 

 respectively. 2 Now it is clear that the augite could not crystal- 

 lise above its proper melting-point of 1200 . The leucite, 

 which followed it, must therefore have crystallised below that 

 temperature, that is at least 220 below its own proper melting- 

 point. Almost any igneous rock affords similar evidence of 

 this lowering of melting-point, or more accurately of freezing- 

 point, of a mineral in the presence of other minerals ; and we 

 can scarcely fail to recognise in this a characteristic property of 

 solutions. 



The order of crystallisation of the constituent minerals of 

 igneous rocks has been the subject of much discussion, as 

 having an obvious bearing on the question of the true nature 

 of rock-magmas and the principles governing their consolida- 

 tion. Rosenbusch has laid down certain generalisations, to 

 which, however, he admits exceptions, and indeed every 

 petrologist knows that the exceptions are very numerous. 

 His rules are two : — (a) The separation of crystals in a silicate- 

 magma follows an order of decreasing basicity ; so that at every 

 stage the residual magma is more acid than the aggregate of 

 the crystals already formed ; (b) The relative amounts of the 

 various compounds present in the magma affect the order of 

 crystallisation, so that in general those present in less amount 

 crystallise earlier. These two rules, if interpreted strictly, 

 would, of course, be inconsistent with one another. The second 

 one is the reverse of what might be inferred from the laws of 

 solutions. It does not appear to be borne out by the facts of 

 petrography, except in so far as concerns the minor accessory 



1 This was clearly realised by Darwin, Geological Observations on Volcanic 

 Islands (1844), P- 122, foot-note. 



2 I take the figure for leucite from Brun. The determinations of Doelter and 

 Joly are lower, though still well above those for augite. The estimation of the 

 melting-points of the silicates is a matter of much practical difficulty ; and, when 

 the melting has been judged merely from change of shape, the temperature found 

 must be almost always too low, the more so since natural crystals necessarily 

 contain impurities. 



