40 
METAMORPHISM. 
BY PROFESSOR D. T. ANSTED_, M.A.^ F.E.S._, ETC. 
»o » ■ 
I N all the natural history sciences there occur series of changes^ 
systematic^, governed by definite laws_, growing out of a 
kind of development_, and leading by successive steps to entire 
modifications,, more or less important, in the general economy 
of natm*e. Thus among insects we have the change from the 
grub to the chrysalis, and from that to the butterfly or beetle. 
Among plants, the conversion of leaves into petals, or petals 
into stamens, may be mentioned as examples. Among 
minerals, the transition from such a rock as vesicular lava, full 
of air blebs, as seen in volcanic countries, to the basalt, whose 
cavities are all filled with crystalline minerals, derived from 
what was once lava, is a similar instance. 
The gradual conversion of one combination of the simple 
elementary substances into another, whether influenced by life 
or not, is a great fact in nature. It is called growth or 
development, when there is fife, but in the mineral world there 
is no such familiar expression for an equally familiar fact. 
The fact of perpetual change is, however, readdy admitted, on 
a moment^ s consideration, as belonging to all nature. It is a 
fact, pregnant with the most important consequences, and 
when the idea of it is carried out logically, it leads us to very 
startling conclusions. 
In geology, and indeed sometimes in botany and zoology, 
this power and habit of conversion is technically called 
metamorphosis {imera, change; /xo/3 0rj, shape). It is a term 
convenient to us when we wish to speak of change of the form 
without essential change of the ingredients of a rock. Thus 
when we take up a specimen of chalk, or of compact limestone, 
capable of being polished as a marble, and on looking carefully 
and microscopically into its structure, find it made up of shells 
and shelly mud, just as we know the mud at the bottom of the 
sea to be accumulating in existing oceans, the geologist 
describes as a metamorphosis the change of form that has taken 
place to convert marine mud into compact limestone. In this 
case the mineral consists of minute particles mixed irregularly 
with larger fragments of shells of various kinds. The mud has 
