§61. RETROSPECT. 119 



as contrasted with the earth itself (see page 35), we may 

 readily conceive that as fissures and inequalities are produced 

 in the varnish of an artificial globe by heat and cold, in like 

 manner the elevation of mountain chains, and the subsi- 

 dence of whole continents, may be occasioned by the 

 expansion or contraction of portions of the earth's crust, 

 from sudden changes in the internal temperature of the 

 globe. 



61. Retrospect. — In this rapid sketch of the geolo- 

 gical phenomena, which an examination of the surface 

 of the earth presents to our notice, I have doubtless 

 dwelt on several subjects familiar to many of my readers. 

 But, as one of our ablest geologists* has remarked, " the 

 teacher of Geology must suppose himself called on to answer 

 questions, both concerning the facts of the science, and the 

 inferences to be deduced therefrom ; and his instructions will 

 be so much the more successful as he takes these questions 

 in the most natural order of their occurrence, and answers 

 them most completely and satisfactorily. In doing this he 

 is not at liberty to neglect even elementary truths, for if 

 these were passed over in compliment to such as have made 

 progress in the science, those for whose advantage he is 

 especially interested, would be called to the unreasonable 

 task of labouring without instruments, and of theorizing 

 without intelligible data." 



From the vast field of inquiry over which our observa- 

 tions have extended, it may be useful to offer a brief sum- 

 mary of the leading principles that have been enunciated, 

 and the facts on which they are founded. By the most pro- 

 found investigations of which the human mind is capable, 

 we learn that our earth is one of myriads of spherical bodies, 

 revolving round certain luminaries ; and that these bodies 

 occur in every variety of condition, from that of a diffuse 



* Professor John Phillips. 



