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Updated: May 9, 2025
Metamorphic Limestone. Origin of the metamorphic Strata. Their Stratification. Fossiliferous Strata near intrusive Masses of Granite converted into Rocks identical with different Members of the metamorphic Series. Arguments hence derived as to the Nature of Plutonic Action. Hydrothermal Action, or the Influence of Steam and Gases in producing Metamorphism.
To what extent some of the metamorphic rocks containing the same minerals as the granites may have been formed by hydrothermal action without the intervention of intense heat comparable to that brought into play in a volcanic eruption, will be considered when we treat of the metamorphic rocks in the thirty-third chapter. Porphyritic granite.
These occurrences are generally held to be due to hydrothermal or geyser action. Then again lodes are often very erratic in their course. Such formations are very common in West Australia. All this has to be considered and taken into account when tracing the run of stone.
In answer to this, it may be observed, that if a number of strata differing greatly in composition from each other be subjected to equal quantities of heat, or hydrothermal action, there is every probability that some will be much more fusible or soluble than others.
But the action was hydrothermal, just such as was seen in course of operation in New Zealand a few years ago when, in the Rotomahana district, one could actually see the growing of the marvellous White and Pink Terraces formed by the release of silica from the boiling water exuding from the hot springs, which water, so soon as the heat and pressure were removed, began to deposit its silica very rapidly; while at the Thames Gold-field, in the same country hot, silicated water continuously boiled out of the walls of some of the lodes after the quartz had been removed and re-deposited a siliceous sinter thereon.
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