§5. STRUCTURE OF THE EARTH. 31 



lessons — to doubt the evidence of his senses, until con- 

 firmed by cautious and patient observation. 



With these introductory remarks I proceed to the con- 

 sideration of the subjects selected for the present discourse, 

 premising, that from the magnitude and diversity of the 

 objects embraced by geology, it is scarcely possible to offer, 

 in the space assigned to a course of popular lectures, even 

 an epitome of the marvels which modern researches have 

 brought to light. This consideration, therefore, must be my 

 apology for the concise manner in which many interesting 

 facts will be noticed ; and I would beg of you to consider 

 that lectures of this kind can only present a general view 

 of the philosophy of geology; that they are calculated to 

 excite, rather than to satisfy, a rational curiosity, and 

 cannot supersede the necessity of study, and of personal 

 investigation. 



5. Structure of the earth. — The globe we inhabit 

 is a planetary orb, about twenty-four thousand miles in 

 circumference, and of a spheroidal shape; its figure being 

 such as a fluid body, made to rotate on its axis, would 

 assume. The mean density is fiye times greater than that 

 of water, the interior being double that of the solid ex- 

 ternal case. From astronomical observations, the original 

 crust of the earth is supposed to have been a superficial 

 coating of solidified matter, produced by the cooling of the 

 surface of an incandescent fluid globe; and on this shell or 

 crust have been slowly accumulated, during the lapse of 

 innumerable ages, the rocks and strata which form the more 

 immediate objects of geological investigation. The thick- 

 ness of the solid crust of the earth is computed at from 800 

 to 1,000 miles, but it is not supposed that this shell is uni- 

 form in extent or density ; on the contrary, it is probable 

 that it is cavernous, and that liquid masses of mineral 

 matter, in various states of fluidity and incandescence, are 

 distributed throughout the interior of the globe. Some of 



