BY H. J. JENSEN, D.SC. G3 



the entrance of the system into a region studded with 

 meteorites. The first is of doubtful possibility, as there 

 seems to be no reason why some regions of space should 

 be hotter than others. The second is a more likely cause* 

 It is easy to understand that if the sun entered a region 

 of space particularly crowded with meteorites or cosmic 

 dust, the first effect on the earth would be a cooling of 

 climates, due to the meteorites intercepting much of the 

 sun's heat before it reached the earth. The second effect 

 would be a rise in the temperature of the sun consequent 

 upon the meteorites falling into it. The objection to the 

 second view is that some of the larger meteorites of such 

 a period should remain fossilised in the geological record, 

 which we cannot state to be the case. 



If there should be any variability in the suix. the 

 periods of hot sun would correspond with periods of slow 

 contraction of our planet and warm climates, like the 

 Cretaceous. Few or no great up-lifts or down-throws such 

 as produce great inequalities en the earth's surface would 

 take place in a period of slow, secular contraction, but the 

 continents w^ould be worn down by rivers, and sediment^ 

 would be piled up in the ocean, and the waters would rise 

 and submerge the lowlands. 



Cold sun periods would accelerate the process of secular 

 contraction. Great wrinkling of the earth's crust would 

 take place. Continent building (epeirogenic) and mountain 

 building (orogenic) uplifts would occur. The super-elevated 

 regions become subject to cold and arid climates, whereas 

 the lowlands acquire moist and cold climates, the 

 diminished intensit}^ of atmospheric circulation causing 

 the atmospheric water to be precipitated excessively on 

 the coastal plains. 



An interesting view on the cause of an ice-age was 

 recently advanced by Professor David. He showed that 

 a drop of 40° in the earth's temperature would move the 

 isotherm of permanent glaciaticn (or snow-line) into the 

 tropics, and would produce just those conditions which 

 are essential for the production of an ice-age in tropical 

 and sub-tropical regions. Such a cold period might not 

 necessarily be accompanied by severe glaciation in polar 

 and temperate regions, for these parts would suffer from 

 drought. There would be no evaporation outside tropical 



