

ALTERATION AND METAMORPHISM 215 
already been made to the fact that rocks which have been 
overflowed or invaded by molten matter are usually more or 
less altered along the line of contact. The changes effected 
by a lava-stream are not particularly conspicuous, and consist 
chiefly of induration, often accompanied in the case of clay 
by a change of colour and the production of prismatic joint- 
ing. The changes caused by intrusive eruptive rocks, 
however, are usually more pronounced. Sometimes, indeed, 
they are of slight importance and confined to the immediate 
proximity of the intrusion; but at other times they may 
extend outwards from the margin of the eruptive mass for 
hundreds or thousands of yards. The extent and intensity 
of the metamorphism depend partly upon the character and 
mass or volume of the intruded rock, and partly upon the 
nature of the rocks invaded. Other things being equal, more 
change is effected by an extensive eruptive mass than by a 
smaller intrusion of the same kind of rock, while certain rocks, 
owing to their composition, are more readily influenced than 
others. 
Some reference has already been made to the kind of changes 
produced upon contiguous strata by basic intrusive rocks—such as the 
conversion of coal into coke, anthracite, or graphite, the crystallisation of 
limestone, the induration of rocks generally, the production of prismatic 
jointing, etc. These and other changes are often exhibited by the 
larger and smaller fragments of sandstone, shale, etc., which have been 
torn from their parent strata and enclosed in an eruptive rock. The 
larger included slabs and blocks are usually much shattered, baked, 
corroded or fused superficially, and even occasionally rendered vesicular 
or scoriaceous. Pieces of felspathic sandstone have been thoroughly 
fused, while fragments of dark shale have been burnt red and baked 
into a hard porcellanite. Similarly, when basalt has caught up and 
enclosed portions of some igneous or schistose rock, such as granite or 
gneiss, these have been either partially or completely fused to a dark 
green or black glass, It is noteworthy that, in the fused portions of 
such included blocks and fragments of various kinds’ of rock, new 
minerals (cordierite, spinel, sillimanite, pyroxene, etc.) have not infre- 
quently been developed. 
Similar changes are effected on the rocks zz s¢¢u along their line of 
contact with sills and dykes of basalt or other basic igneous rock. Fusion, 
however, is confined to the actual line of contact, while induration and 
other changes may extend outwards for many feet or yards, the width of 
the metamorphosed belt being dependent on the volume of the eruptive 
mass, and to a large degree also upon the character of the surrounding 







