HKB
carboniferous strata in England, the Upper and Lower Silurian in East Canada, and the oldest clay-slates in Norway, all contain as much alkali as is generally present in metamorphic rocks.
Another objection has been derived from the alternation of highly crystalline strata with others less crystalline. The heat, it is said, in its ascent from below, must have traversed the less altered schists before it reached a higher and more crystalline bed. 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. Some, for example, will contain soda, potash, lime, or some other ingredient capable of acting as a flux or solvent; while others may be destitute of the same elements, and so refractory as to be very slightly affected by the same causes. Nor should it be forgotten that, as a general rule, the less crystalline rocks do really occur in the upper, and the more crystalline in the lower part of each metamorphic series.
[1] Keilhau, Gæa Norvegica, pp. 61-63.
[2] Geol. Manual, p. 479.
[3] Syst. of Geol., vol. i, pp. 210, 211.
[4] See Lyell, Quart. Geol. Journ., vol. i, p. 199.
[5] Phil. Trans., 1804.
[6] Daubrée, Sur le Métamorphisme. Paris, 1860.
[7] See Principles, Index, “Carbonated Springs,” etc.