94 HISTORY OF THE EARTH. 
of the present clay, that basalt is of igneous origin, and in arriving 
at this conclusion, they have been led to include with it, granite 
and the other unstratified rocks. 
51. But how have these last been produced? In what particu- 
lar way have they after their formation been heaved out of their 
beds, and placed at great elevations above the level of the sea ? 
Is there any analogy either intimate or remote, between the 
causes and circumstances that have formed beds of lava upon the 
surface, and those which created and raised into its present posi- 
tion, the block of granite on which the University stands? On 
these points the opinions of geologists are widely discordant. 
1. It has been supposed by some, that there is a close resem- 
blance in the formation of beds of lava and masses of granite, the 
causes to which they owe their origin, and the mode of their ac- 
tion, being nearly the same for both : That they are the result 
of chemical changes which are constantly proceeding within the 
crust of the earth, and which either operate unseen and in silence, 
untii a force is accumulated that is superior to the resistance to 
be overcome ; or acquire from time to time new activity, because 
fresh masses of matter are brought into contact with each other. 
In either case, there is a sudden and irresistible action, by which 
long ranges of mountains are thrown up in the course of a few 
weeks, or months, when the disturbing force appears to be ex- 
hausted and sleeps for ages. The history of the earth, according 
to this hypothesis, is made up of brief paroxysms of violence and 
convulsion, and long intervals of repose. 
2. Other geologists have represented that the amount of geo- 
logical change has been in all ages pretty nearly the same ; that 
the existing continents have been raised from the deep by a suc- 
cession of movements, each so small as to have escaped notice at 
the time of its occurrence, and which are still continued. Thus 
some parts of the coast cf Sweden are said to be rising at the 
present day, but so slowly, that the fact is ascertained only by a 
comparison of observations made at intervals of from fifty to one 
hundred years. That there has been no change of the relative 
level of land and water around the shores of the Mediterranean, 
during many centuries, is proved by the fact that the stairs for de- 
scending to the water's edge, and the landing-places there, which 
were constructed by the ancient Greeks and Romans, are in use 
at the present day. 
3. But neither the paroxysmal nor the secular hypothesis as- 
signs with precision and certainty the mode by which the effects 
supposed by it are accomplished. Granting that the causes of 
chemical change are active in the interior of the earth ; it is not 
apparent in what way they can operate to produce the protrusion 
of the rocks and the elevation of the strata. We cannot see why 
they should even have any tendency to produce such effects. 
And even if they are supposed only to modify the level of the 
existing continents, by causing the depression of one point, 
