
Part VII. §i] METAMORPHISM. 571 
type and to a totally different order of physical conditions. Natural, 
however, as this conclusion may be, those who adopt it probably 
seldom realise to what an extent it rests upon mere assumption. 
Starting with the supposition that the crystalline schists are the 
result of geological operations that preceded the times when 
ordinary sedimentation began, it assumes that they belong to one 
great early geological period. Yet all that can logically be asserted 
as to the age of these rocks is that they must be older than the 
oldest formations which overlie them. If in one region of the globe 
they appear from under Cretaceous, in another below Carboniferous, 
in a third below Silurian strata, their chronology is not more 
accurately definable from this relation than by saying they are re- 
spectively pre-Cretaceous, pre-Carboniferous, and pre-Silurian. They 
may all of course belong to the same period; but where they occur 
in detached and distant areas their synchronism cannot be proved. 
To assert it is an assumption which, though in many cases irresistible, 
ought not to be received with the confidence of an established truth 
in geology. 
In the investigation of the problem of the crystalline schists much 
assistance may be derived from a study of the localities where a 
erystalline and foliated structure has been superinduced upon 
ordinary sedimentary rocks—where, in fact these rocks have actually 
been changed into schists, and where the gradation between their 
unaltered and their altered condition can be clearly traced. Accord- 
ingly the following pages of this Part will be devoted to an exami- 
nation of the salient features of metamorphism and metamorphic 
rocks. 
At the outset some caution must be employed as to the use of 
the terms “ metamorphism” and “ metamorphic.” It is obvious that 
we have no right to call a rock metamorphic unless we can distinctly 
trace it into an unaltered condition, or can show from its internal 
composition and structure that it has undergone a definite change, or 
can prove its identity with some other rock whose metamorphic 
character has been satisfactorily established. Further, it must be 
remembered that in a certain sense, all or nearly all rocks may be 
said to have been metamorphosed, since it is exceptional to find any, 
not of very modern date, which do not show, when closely examined, 
proofs of having been hardened by the pressure of superincumbent 
rock and altered by the action of percolating water or other daily 
acting metamorphic agent. Even a solid crystalline mass which, 
when viewed on a fresh fracture with a good lens, seems to 
consist of unchanged crystalline particles, will usually betray under 
the microscope unmistakable evidence of alteration. And this 
alteration may go on until the whole internal organization of the 
rock, so far at least as we can penetrate into it, has been readjusted, 
though the external form may still remain such as hardly to indicate 
the change, or to suggest that any new name should be given to the 
recomposed rock, Among many igneous rocks, particularly the 
