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DYNAMICAL GEOLOGY. 



ganizing force was lateral pressure or thrust ; and that there was a 

 pressing side, and a more feebly pressing, or virtually resisting side, 



II. Fundamental Agency in Mountain-making. 



The organizing force in the sphere having acted by lateral pressure 

 or thrust, what now is the source of this pressure ? What condition 

 in the earth's interior has caused that lateral pressure should have 

 been evolving effects through all past time ; should have worked out 

 the earth's grander features, making even its mountain chains? The 

 answer to this question involves a consideration of the nature of the 

 earth's interior ; this done, we are prepared to consider the working 

 methods of the force engaged, and the special steps in the making of 

 mountains and continents. 



1. Condition of the Earth's Interior. 



Three opinions respecting the earth's interior have prominence: 

 First, that it is wholly solid to the centre ; secondly, that it has a 

 solid centre and crust, and a liquid or pasty layer between ; thirdly, 

 that it has a solid crust and liquid interior — the interior either per- 

 fectly liquid ; or wholly in a pasty state ; or pasty above and liquid 

 below. 



Arguments on the subject have been based on the following consid- 

 erations : — 



1. Considerations connected with the Attraction of the Earth by 

 the Sun and Moon. — (1.) The precession of the equinoxes is due to 

 the attraction of the sun and moon on the protuberant equatorial por- 

 tion of the globe; none would exist were it a perfect sphere. Pro- 

 fessor W. Hopkins sought to ascertain the difference between the 

 present amount of precession and what it should be were the interior 

 a liquid moving freely beneath a thin crust. In such a case the force 

 producing the precession would not have to pull around the liquid 

 interior portion, on account of its (assumed) freedom of movement, 

 and the precession should therefore be greater than with a solid globe. 

 Finding, by his calculations on the basis of a solid or nearly solid 

 globe, that the precession obtained agreed quite closely with the ex- 

 isting amount, Professor Hopkins concluded that the solid crust must 

 have a thickness of at least 800 or 1,000 miles. 



But, as pointed out by Hennessy, and later by Delaunay, the as- 

 sumption by Professor Hopkins of a condition of perfect freedom of 

 movement in the liquid of the interior is unwarranted. For all 

 liquids move with some friction among the particles, and the most 

 liquid of lavas, with a very large amount of it ; and if there was a 

 pasty state for a considerable depth beneath the crust, so that, for a 



