734 DYNAMICAL GEOLOGY. 



that these transverse or latitudinal oscillations, while most promi- 

 nent in the higher latitudes, also had their lower-latitude effects ; 

 that they resulted in deepening the transverse seas that divide the 

 northern and southern continents. For the occurrence of Post- 

 tertiary animals on Sicily, Malta, and Gozo, m the Mediterranean, 

 similar to those of Africa, is strong testimony, as some writers have 

 observed, that these islands were joined to the mainland on the 

 south even in the last period of geological history, when the Age 

 of Man was opening. The barrier coral reefs and coral islands of 

 the East and West Indies indicate as recent depressions in those 

 inter-continental seas. Even the origin of the British Channel, 

 as some have thought, may be of the same period, if the identity 

 of European and British Post-tertiary mammals is not to be ex- 

 plained by independent creations in the now disjoined lands. 

 Moreover, the Pacific Ocean, within the tropics, has registers of 

 subsidence all over it, in its coral islands. 



3. Deductions from the courses of the reliefs and outlines. 



The prevalent northeast and northwest courses of trends, the 

 curves in the lines varying the direction from these courses, and the 

 dependence of the outlines and feature-lines of the continents and 

 oceanic lands upon these courses, are the profoundest evidence of 

 unity of development in the earth. Such lines of uplift are lines 

 of fracture, or lines of weakest cohesion ; and therefore, like the 

 courses of cleavage in crystals, they show by their prevalence some 

 traces of a cleavage-structure in the earth, — in other words, a ten- 

 dency to break in two transverse directions rather than others. 



Such a cleavage-structure would follow from the nature of the 

 earth's crust. The crust has thickened by cooling until now scores 

 of miles through, and very much as ice thickens, — by additions to 

 its lower surface. Ice takes on a columnar structure, perpendi- 

 cular to the surface, in the process, so as often to break into columns 

 on slow melting. The earth's crust contains as its principal ingre- 

 dient the cleavable mineral feldspar ; and as the crystals of this 

 mineral usually take a parallel position in a granite, so that the 

 granite of a quarry has its directions of easiest fracture, so it might 

 be in the cooling crust. This appears the more probable when it 

 is considered with what extreme slowness the thickening of the 

 crust has gone on, and the immeasurable length of time it has 

 occupied. 



There are three elements at the basis of the earth's features. 

 First, a geographical one, — the positions and extent of the con- 



