38 Day, etc. — Determination of Mineral 



the diabase in the form of inclusions is more than half fused 

 at 1150°, but shows no fusion at 1025°. (3) The inclusions as 

 actually found show no indication of fusion or flow. As indi- 

 cated by their present properties under atmospheric pressure, 

 there is therefore a gap of at least 100° between the maximum 

 temperature to which the arkose inclusions could have been 

 subjected, and the minimum temperature at which the diabase 

 will flow. The most probable agent which possesses the power 

 to bridge this gap is the water content of the diabase magma, 

 of which the surrounding rocks show abundant evidence. At 

 the time of the intrusion water was undoubtedly present and 

 served to lower considerably the fusion temperature of the 

 diabase. 



It is evident, then, that we are by no means at the end of 

 the geological problem of the relative volumes of crystalline 

 and molten rocks when we have determined their volume 

 under the conditions of pressure and composition in which we 

 now find them at the surface of the earth. The effect of the 

 volatile components, now present only in traces or perhaps 

 entirely absent from the rock, is still to be taken into account. 

 The effect of dissolved water, for instance, on the volume of 

 liquid diabase as given in the preceding pages, is unknown, 

 although an amount sufficient to produce large changes in 

 melting temperature and viscosity will probably be found to 

 produce relatively small changes in volume. Data such as we 

 have presented are absolutely necessary, however, as a basis 

 upon which to build further knowledge of the volatile compo- 

 nents, when methods are worked out for including them. 



Summary. 



After reviewing existing and rather conflicting data on the 

 change in volume on fusion of various rocks, we have described 

 apparatus by which the volume of metals and of solid and 

 liquid silicates can be determined from 250° to 1600°. The 

 sensitiveness of the method is from O03 to 0"2 per cent. The 

 accuracy varies with the metal used, the temperature, and the 

 size of the sample, but is usually between 02 and 05 per cent. 



The fundamental constant is the expansion coefficient of arti- 

 ficial graphite, which is given by the formula 10 6 = 0*55 4- 

 0-0016 t. In Table I (p. 14) are given the volumes of tin, lead, 

 and the tin-lead eutectic in the liquid state at various tem- 

 peratures. 



Volume measurements on quartz up to 1600° show a 

 dilatation which increases rapidly as the inversion point of 

 575° is approached. Above this temperature quartz contracts 

 slightly. At about 1300° there begins a second and (under these 



