THE STKUCTUKE OF HOCK MASSES. 
237 
soften the rocks), under a slight pressure of from about seven 
to twelve pounds per square inch, that they became totally 
changed in structure and converted into an aggregate of 
finely developed crystalline folise of a brilliant white or greenish 
colour, identical with talc ; in fact, a talc schist. Under similar 
circumstances, if protected from oxidation — as they contain 
some iron in the state of protoxide — ordinary clayslates were 
converted into rocks possessing a beautiful parallel structure, 
resembling gneiss so closely, that some of the hand specimens 
could not be distinguished by the eye from parts of the same 
clayslate altered at its points of contact with eruptive rocks 
in nature. In the first of these cases, mere re-crystallisation or 
molecular re-arrangement in the solid mass will explain the 
change ; but, in the second, chemical action also has evidently 
come into play in re-arranging the chemical elements of the 
clayslate into other mineral forms not pre-existing in the slate. 
The effect of the heat being to expand and render the pores of 
the rock more open, doubtless admits of the molecules re- 
arranging themselves and crystallising in the still solid rock, 
whilst at the same time the superincumbent pressure tends to 
force the crystals to shoot out or develope themselves in one 
direction only, i.e. at right angles to the pressure. 
These experiments, and the formation of the well-known 
Eeaumur’s porcelain, show how such crystalline structure can 
be developed in solid bodies after their perfect solidification 
without any return to a fluid or molten condition ; and the 
annexed woodcut, taken from Keilhau, 
Fig. 4. which represents an appearance in 
the gneiss of Jomfrueland, an island 
on the southern coast of Norway, can 
only be explained on the assumption 
that the direction of the lines of folia- 
tion in the rock (which are represented 
by the dotted lines) had been deter- 
mined subsequent to its complete 
consolidation, since it will be noticed 
that the subordinate bed of dark 
liornblendic character has been dis- 
located by the fault ab, without any 
corresponding disturbance in the lines of foliation. 
In the present state of science it would be premature to say 
more as to the probable causes of foliated structure ; the hope 
may, however, be expressed, now so much attention is devoted 
to geological research, that the study of this interesting although 
abstruse subject may no longer be neglected. 
Striation , or that structure which is due to the presence of 
what are termed the striae of fusion, has frequently been called 
