190 MSH AIBIE (6, SRO SV BICIL 
heated and plastic condition and enclosed in a more rigid shell 
which contracts as it cools, a force is brought to bear on the 
plastic interior which tends to squeeze it out whenever an oppor- 
tunity is afforded. The idea is not that the crust of the earth is 
a cold and solid shell enclosing a molten interior, the surface of 
contact between the two being sharply defined; but rather that 
the highly heated and plastic interior passes by insensible grada- 
tions into a cooler and more solid exterior, the outer surface of 
which is cold and rigid. The crust is still losing heat and con- 
sequently contracting and thus brings a pressure to bear not only 
on the interior mass but on the material forming the crust itself. 
On account of irregularities and unequal cooling, regions well 
within what may be termed the crust, may still be in a molten 
condition. Such bodies of plastic material may differ from the 
rocks enclosing them, as well as vary in composition among 
themselves, by reason of having reached different stages in the 
process of change from a molten to a solid condition. Here 
again we are departing from observed conditions, and have only 
the imagination to guide us, as there are no tests available by 
which the truth of our conceptions can be verified. 
Under the conditions assumed, when a fissure is formed in the 
earth’s crust, the deeply seated fluid or plastic material within, is 
forced out and becomes more and more fluid as it rises and pres- 
sure is relieved, unless this tendency is more than counteracted by 
the decrease of fluidity due to loss of heat. When such fissures 
open a way to the surface volcanic phenomena ensue. If the 
fissures do not reach the surface but terminate in horizontally 
stratified rocks, the molten material which rises in them may 
spread more or less horizontally between the strata and form 
sheets, or exert its force locally and cause a dome to rise, accord- 
ing to its viscosity, the depth below the surface, etc. 
Nothing less than the force produced by the contraction of a 
cooling globe seems adequate to account for the results to which 
attention has been directed. The slowness with which the earth 
has lost heat and the consequent slowness with which contrac- 
tion has acted on the still plastic interior or still plastic reservoirs 
