THEIR CHRONOLOGICAL ST 



not to the mere volcano alone that we mi 

 full effects of vulcanic energy. The earthquake 

 motions alluded to in a former Sketch (No. 3) are in like 

 manner ever busy in moulding and modifying the earth's 

 exterior the former fracturing and fissuring the rocky 

 crust, and giving tenfold significance to the discharges of 

 the volcano, the latter silently elevating the sea-hed into 

 dry land, or submerging the dry land beneath the waters. 



Such is a rapid sketch of the Post-Tertiary or Quaternary 

 formations, which, though recent and superficial to us, will 

 become old and deep-seated to the observers of future ages. 

 Scattered over the land, and spread out under the waters, 

 they are not only everywhere present, but of all dates, 

 from the accumulations of the current century back to those 

 that mark the close of the glacial epoch. Indeed, the main 

 difficulty connected with them is to assign their respective 

 dates, and fix a scale of chronology that will be at all gene- 

 rally applicable. Superposition can only be applied in very 

 limited cases; and mineral composition is of little avail, 

 as the older are often as loose and unconsolidated as the 

 younger. The only satisfactory test is fossils, striking back 

 from the existing flora and fauna of any locality to those 

 that have been removed from that locality, and from these 

 back to such as have become totally extinct. In this way 

 we may speak of Upper, or those containing existing plants 

 and animals ; of Middle, or those characterised by plants 

 and animals locally extirpated j and Loicer, or those marked 

 by organisms now wholly extinct. As regards Man, they 



long, by 12 or 15 in its greatest breadth, and from 20 to 600 feet in 

 thickness, according to the nature of the ground ; while Dr Coan esti- 

 mates the discharge of Mauna Loa (one of the Sandwich Island volcanoes) 

 in August 1855, at 70 miles long, with a varying width from 1 to 5 miles, 

 and from ten to several hundred feet in thickness. 



