| OCR Text |
Show 132 GEOLOGY OF THE HIGH PLATEAUS. to- see if there be anything in the physical properties of the rocks to justify such a hypothesis. We can represent this best by a graphic expression of their physical properties regarded as functions of temperature and acidity. Let the axis of abscissas, Plate 4, represent the proportions of silica characteristic of the various groups of volcanic rocks, the figures along that axis representing percentages from 40 to 80. Let the ordinates represent, first, the density of the rocks in the cold state. Considering now any one variety of rock, take the point on the axis of abscissas corresponding to its percentage of silica, and erect an ordinate proportional to its density. For all the varieties of rocks construct ordinates in the same manner and join their upper extremities. On the assumption that the density is rigorously correlated to the percentage of silica, a curve would be constructed representing the density as a definite function of the silica. This assumption, however, is not strictly true, being subject, indeed, to notable variations; yet in a general way it is more or less an approximation to the truth. The anomalies will be adverted to in the sequel. It is known that the rocks of the basic and sub-basic groups are when cold considerably more dense than the.average of the foliated rocks, and the same is true of some of the sub-acid rocks, and according to the. doctrine heretofore laid down such rocks could not be erupted at all were it not for the fact that when intensely heated and liquefied, their density is notably diminished and reduced below that of the strata which overlie them. Hence the more basic the rock, the more it must be heated to reach an eruptible density. The ordinates, then, may be used to represent ' the relative increase of temperature which must supervene in order to render the rocks light enough to reach the surface, and as these increments of temperature are directly proportional to the density of the rock, the same curve may (in the absence of fundamental constants) be used to express the increments of temperature required by the various rocks to reach an eruptive. density. Again, let the ordinates represent the relative melting temperatures of the various sub-groups, the assumption still being that the fusibility is a definite function of the proportion of silica. This assumption is probably subject to still wider variations than that which postulates a dependence of |
