RESULTS OF THE EARTH'S CONTRACTION. 735 



THE EARTH A COOLING GLOBE: ITS CON- 

 SEQUENCES. 



As the globe has cooled from fusion, it has been through all time a 

 contracting globe ; and this contraction of the crust has been the chief 

 agency in determining the evolution of the earth's surface-features, 

 and the successive phases in its long history. 



I. GENERAL CONSIDERATIONS. 



1. Seat of the Organizing Agency of Contraction. — It is stated on 

 page 146, that solidification probably began, as suggested by Hopkins, 

 at the earth's centre, and as a consequence of pressure ; that the tem- 

 perature of the globe in which pressure would produce this result was 

 reached long before a crust began to form from external cooling ; and 

 that, when the crust had formed, the globe consisted of (1) a solid nu- 

 cleus, which had solidified from the centre outward : (2) a crust, which 

 had solidified from the surface inward ; and (3) a layer of plastic rock 

 between the two, which, through continued cooling, would tend to be- 

 come united ultimately with the latter. The earth's crust could not 

 have undergone flexures, unless it were lying upon a bed of liquid 

 rock, that could yield before it; and, if the globe is now essentially 

 solid throughout its mass, there must have been in past time, at least, 

 a more or less complete layer of plastic rock-material, such as this 

 view of Hopkins sup[)oses. The subsidences and elevations have 

 affected areas of vast extent at once ; and even in the Quaternary, or 

 Quaternary and Tertiary, occurred the coral-island subsidence in the 

 Pacific, which moved the bottom of the ocean for a breadth of 5,000 

 miles. The fire-seas beneath the crust must hence have been of great 

 extent. 



In accordance with the above, the organizing agency of contraction 

 was confined mainly to the crust, of which the supercrust is an 

 upper transformed portion (p. 147) ; and there was enough of plastic 

 rock beneath this crust to have allowed of all the bendings the crust 

 has experienced. 



2. The Force resulting from Contraction. — The crust which should 

 form over a melted sphere, as it cooled, would have the size the sphere 

 had at the time. As it thickened downward, by the continued cooling, 

 the added portions would contract, since the density of the solidified 

 rock is at least eight per cent, greater than that of the liquid, and this 

 would occasion lateral pressure throughout the crust, which would in- 

 crease as the cooling and thickening continued. A yielding some- 

 where would finally become an inevitable result. 



