PERMANENCE OF VOLCANIC ACTION. 63 



centres, we know next to nothing, and are merely on the 

 threshold of the inquiry. 



Little, however, as we know either of the cause or of 

 the course of vulcanism, we can readily perceive its func- 

 tion, and behold in it one of the great means by which the 

 crust of the globe is held in equilibrium, and by which the 

 diversity and variety of its surface is maintained. Were 

 there no adequate force acting from within, the powers of 

 waste and degradation from without would in time reduce 

 the surface to one dreary monotony of level, incompatible 

 with that diversity of condition and of life which appears 

 to be one of the great aims of creation. But just as the 

 meteoric and aqueous forces wear and waste from without, 

 so the vulcanic renovate and upheave from within; and 

 thus the rocky crust is held in perfect equipoise, and its 

 surface diversified by all that irregularity of hill and val- 

 ley, of table-land and plain, which is indispensable to variety 

 -in the plant-life and animal-life by which it is adorned. 

 Wherever large expanses of the earth's surface, like the 

 prairies and pampas of America, are characterised by same- 

 ness of condition, there is a consequent want, of variety in 

 their vegetable and animal existences ; but as the great de- 

 sign of Creation seems to be variety in space as well as 

 variety in time, this uniformity of surface is incessantly 

 broken up by the operations of the earthquake and volcano. 

 Locally disastrous as may be the throes of the one or the 

 discharges of the other, we thus behold in each a necessary 

 part of the world's mechanism, and powerful in proportion 

 to the work it has to accomplish. 



It has been frequently discussed, and with some is still 

 a question, whether this power of internal vulcanism be 

 steadfast or declining, and whether it did not manifest 

 itself with greater intensity during the earlier geological 

 periods? Locally we may perceive that it has ceased in 



