Ixxil PROCEEDINGS OF THE GEOLOGICAL socieTy. [May 1902, 
historian in this matter,——of all those theories that would explain 
the crystalline schists by reference to physical conditions essentially 
different from those which have prevailed during successive geological 
periods. 
Most of the earlier ideas were based on the assumption that the 
erystalline schists form a natural group, whose origin must therefore 
be explained by some comparatively simple hypothesis; but the de- 
tailed researches of the later decades have shown that this assumption 
is wrong. It is now generally recognized that these rocks present 
us with structural and petrographical problems of great complexity. 
Excluding the gneisses of igneous origin, which form a large portion 
of the group, I believe that the other rocks may for the most part 
be classed as rocks of either mixed or metamorphic origin. I propose, 
therefore, to limit my remarks almost entirely to the growth of 
ideas on the subject of metamorphism. : 
Hutton’s ‘Theory of the Earth’ (vol. i, pp. 375-76) contains 
this remarkable passage. I have quoted it before, but it will bear 
repetition :— 
‘Tf, in examining our land, we shall find a mass of matter whieh had evidently 
been formed originally in the ordinary manner of stratification, but which is 
now extremely distorted in its structure, and displaced in its position,—which 
is also extremely consolidated in its mass, and variously changed in its com- 
position,— which therefore has the marks of its original or marine composition 
extremely obliterated, and many subsequent veins of melted mineral matter 
interjected ; we should then have reason to suppose that here were masses of 
matter which, though not different in their origin from those that are gradually 
deposited at the bottom of the ocean, have been more acted upon by sub- 
terranean heat and the expanding power, that is to say, have been changed ina 
greater degree by the operations of the mineral region. If this conclusion 
shall be thought reasonable, then here is an explanation of all the peculiar 
appearances of the Alpine schistus masses of our land, those parts which have 
been erroneously considered as primitive in the constitution of the earth.’ 
Hutton made no attempt to frame a classification of rocks in 
accordance with his theory; but this was done some forty years 
later by Lyell, who crystallized the main ideas which are expressed 
in this paragraph in the one word, metamorphic. 
Modern ideas on the subject of metamorphism have been slowly 
elaborated by a long course of observation and experiment. Sir 
James Hall was the first to realize the value of experiment for the 
purpose of verification. Hutton’s theory required that ordinary 
limestone should pass into crystalline marble under the combined 
influence of heat and pressure. Hall enclosed the powder of lime- 
stone in hermetically sealed tubes and proved that this actually 
