Modern Igneous Rocks and the Primary Formations. 115 
it to enter into new combinations. ‘Tourmaline is one of the 
combinations which in the cases cited may have absorbed, or 
rather, have used up the iron, and as this mineral contains iron 
in the state of a protoxyd, we perceive that this deoxydation must 
actually have taken place. 
Other examples might be cited from the sandstone and trap 
region of Massachusetts and Connecticut, but they illustrate no 
new principles. Besides tourmaline and epidote, garnets have 
been observed adjoining some foreign dykes. 
i have purposely avoided mentioning facts collected during the 
cruise of the Expedition, but will cite one example in farther 
illustration of the principles here supported—the one which first 
suggested these views to my mind. It occurs on a small island 
at the mouth of Hunter River in New South Wales. A dyke 
of basalt, only eight feet wide, cuts vertically through the coal, 
clays and sandstone of the coal formation. The coal for six 
or eight feet is deprived of its bitumen, and as some of the layers 
contain considerable clay, it is baked to a hard black rock, con- 
taining masses of coal resembling charcoal. Beyond this distance 
it is unaltered. The soft clays are changed toa bluish chert, 
like flint in hardness and fracture, as far as the extremity of the 
island, which is about eighty yards from the dyke. ‘The sand- 
stones are also baked and hardened, but less distinctly at this dis- 
tance than the clays. Such are the facts, and do we need other 
evidence that heated waters can and actually do alter rocks? 
Were the heat of the dyke conducted through the rocks from the 
dyke, to such an extent as to turn clays into flint eighty yards off, 
the coal surely ought to have been burnt or deprived of its bitu- 
men toa greater distance than two or three yards? Moreover 
the clays show no evidence of fusion even in the vicinity of the 
dyke. Instead of the very intense ignition required to bake 
rocks so far from its source by conduction alone, without the in- 
tervention of water, a comparatively low temperature will heat 
the mobile waters sufficiently to produce the same effect. 
Before applying these principles to granitic rocks, and the as- 
sociated schists, I would request your attention to another mode 
in which heated waters modify the rocks that come under their 
influence. 
We know that water intensely heated, will dissolve various 
earths and earthy compounds that are untouched by it when cold, 
