144 Things not generally Known. 



uniformity of factor ; and until we can bring elements of cal- 

 culation as exact as those of astronomy to bear on geological 

 chronology, it will be better to regard our " eras" and " epochs" 

 and " systems" as so many terms, indefinite in their duration, 

 but sufficient for the magnitude of the operations embraced 

 within their limits. Advanced Textbook of Geology, by David 

 Page, F.G.S. 



M. Rozet, in 1841, called attention to the fact, that the 

 causes which have produced irregularities in the structure of 

 the globe have not yet ceased to act, as is proved by earth- 

 quakes, volcanic eruptions, slow and continuous movements 

 of the crust of the earth in certain regions, &c. We may, 

 therefore, yet see repeated the great catastrophes which the 

 surface of the earth has undergone anteriorly to the histori- 

 cal period. 



At the meeting of the British Association in 1855, Mr. Hop- 

 kins excited much controversy by his startling speculation 

 that 9000 years ago the site on which London now stands was 

 in the torrid zone ; and that, according to perpetual changes 

 in progress, the whole of England would in time arrive within 

 the Arctic circle. 



CURIOUS CAUSE OF CHANGE OF LEVEL. 



Professor Hennessey, in 1857, found the entire mass of rock 

 and hill on which the Armagh Observatory is erected to be slightly, 

 but to an astronomer quite perceptibly, tilted or canted, at one season 

 to the east, at another to the west. This he at first attributed to 

 the varying power of the sun's radiation to heat and expand 

 the rock throughout the year; but he subsequently had reason 

 to attribute it rather to the infiltration of water to the parts 

 where the clay-slate and limestone rocks met, the varying 

 quantity of the water exerting a powerful hydrostatic energy 

 by which the position of the rock is slightly varied. 



Now Armagh and its observatory stand at the junction of 

 the mountain limestone with the clay-slate, having, as it were, 

 one leg on the former and the other on the latter ; and both 

 rocks probably reach downwards 1000 or 2000 feet. When 

 rain falls, the one will absorb more water than the other ; both 

 will gain an increase of conductive power ; but the one which 

 has absorbed most water will have the greatest increase, and 

 being thus the better conductor, will draw a greater portion of 

 heat from the hot nucleus below to the surface will become, in 

 fact, temporarily hotter, and, as a consequence, expand more 

 than the other. In a word, both rocks will expand at the wet 

 season; but the best conductor, or most absorbent rock, will expand 

 most, and seem to tilt the hill to one side; at the dry season it will 



