Page:The Journal of geology (IA journalofgeology11893univers).pdf/151
acid is entirely expelled only from very fine-grained mixtures of calcareous and ashy materials: approaching the contact, the complete decomposition of the calcite is found to extend to successively coarser-grained rocks. Another line of inquiry is offered by the texture of the metamorphosed rocks themselves, of whatever lithological nature, in a district of metamorphism surrounding a large igneous intrusion. The size of the individual crystals of secondary minerals increases towards the contact with the intrusive rock: this may be taken to indicate that the migration of material within the mass of a rock undergoing metamorphism has more latitude when the temperature is higher. For various reasons, however, it would be unsafe to found numerical results upon such observations. The crystals of certain metamorphic minerals attain to considerable dimensions by virtue of their power of enclosing a large amount of foreign material; others, again, can apparently push aside solid impurities to make room for their own growth. The texture of the metamorphic rocks examind is still, however, in general accord with the conclusions reached by other methods of inquiry.
The question naturally arises whether the limit of migration of material is the same for different substances. On this point we have but little information. Among the various types of "spotted" rocks described in aureoles of metamorphism is one in which the spots are simply spaces free from the secondary brown mica abundant in the general mass of the metamorphosed rock. Since the iron compounds in the rock must originally have had a generally uniform distribution, the phenomena of the spots indicate a movement of ferrous oxide, and the radius of the spots gives a measure of the extreme limit of such movement. In the cases examined this is about one-twentieth of an inch, and we may infer that the greatest distance of migration of ferrous oxide has been about the same as that of silica at a similar temperature.
Not to insist unduly upon precise estimates, these and similar observations certainly tend to show that in thermal metamorphism no interchange of material takes place except between