OCR Text |
Show 138 geology; of the high plateaus. ». horizontally in the amount of rise as we pass from point to point. It may also rise more rapidly in the lower part of the field than in the upper; and as between many fields, local circumstances may accelerate beyond the mean rate the fusion and expansion of one class of rocks or retard the same effects in others. Thus, while there is a normal or typical order of eruptions, it may become liable to not infrequent exceptions arising from want of exact homogeneity of conditions. There are several sub-groups of rocks which present difficulties somewhat greater and have the appearance at present of being somewhat anomalous. These â- are principally quartz-propylite and ' quartz-andesite or dacite. These rocks are much more siliceous than the other members of the groups to which they are mineralogically most nearly allied, being about as siliceous as the more acid trachytes. They have apparently had their epochs of eruption coevally with the hornblendic members of their respective major groups, while according to the theory their epochs should have fallen much later. I am unable to harmonize these apparent anomo-lies with the main theory upon any considerations which at once carry with them a conviction of intrinsic probability and an obvious reason for their exceptional relations. They are comparatively rare rocks, and do not occur in very extensive masses; their physical constitution and properties are much less known than their chemical and mineralogical. Their inferior bulk, however, does not break the force of the anomaly if it be real. Considerations like the following, suggest themselves: The theory assumes that the physical properties (density and fusibility) have a definite relation and dependence upon the proportion of silica which a rock contains. Although this is approximately true, it is in all probability not rigorously so, and indeed the probabilities, so far as fusibility is concerned, are that the variations from definiteness in the dependence of fusibility upon the percentage of silica are in some cases very notable, though these variations may not impair, the general law as an approximate expression of the truth. In spite of their high percentage of silica, therefore, these rocks may turn out to be exceptional in having a degree of fusibility corresponding very closely to that of the hornblendic members of the major groups to which they belong. While, therefore, we cannot claim the dacites and |