104 SCIENCE PROGRESS 



minor cycles of glaciation (orthodox or Draysonian) begun and ended with 

 the superimposition of a greater rhythm ? Major Marriott himself {Changes 

 of Clhnate, p. i8) has to admit that the recurrent glacial conditions worked 

 up to a maximum of intensity (in the " boulder-clay " glaciation) and then 

 again decreased, and has to introduce an irregular rhythm of alteration in 

 the shape of the earth's orbit with varying periods of hundreds of thousands 

 of years to explain this. Furthermore, how do these superimposed cycles 

 link on to the great Permian equatorial and other glaciations of the geological 

 past ? 



A further objection to the astronomical cycles is their short period, which, 

 under the most favourable conditions, would not allow the known spread of 

 the Pleistocene ice sheets, their oscillation about their extreme limits, and 

 their final retreat and extinction, within the time limits proposed. For 

 example, Chamberlin and Salisbury state that the Labrador and Keewatin 

 ice sheets appear to have pushed out from their centres about i,6oo and 1,500 

 miles respectively ; but allowing only 1,000 miles of advance at the rate of 

 one foot per day (an estimate much beyond the probabilities) and not including 

 halting and retreating stages, it would take more than 14,000 years for the 

 ice margin to reach its observed limit of extension. If the safety factor of 

 500 miles be included, a corresponding increase in time must be allowed. 

 Similar figures must obtain for the European Pleistocene glaciation. Even 

 within the limits of the augmented Draysonian cycle it is difficult to find 

 time for the spread of the ice sheets, for the known oscillations of their margins 

 about extreme positions, and for their final retreat and disappearance. 



Finally, is it at all certain that an increase in the obliquity of the ecliptic 

 is competent to bring about glacial conditions ? Major Marriott dismisses 

 this question very inconclusively in a paragraph {Changes of Climate, p. xi), 

 and states that the question has never been subjected to discussion among 

 physicists. This, however, is incorrect, as reference to chapters xiii and xiv 

 in W. B. Wright's Quaternary Ice Age (1914) will show. In reference to the 

 hypothesis of glaciation by change in the obliquity of the ecliptic, Wright 

 tabulates (p. 300) the calculations of Meech regarding the amount of heat 

 received on each 10 degrees of latitude when the obliquity of the ecliptic 

 was near its maximum value 10,000 years before 1800, and remarks, " It is 

 considered that these figures demonstrate that no marked climatic changes 

 can be produced in this way." 



Geologists are coming to believe that the great climatic variations of the 

 past are based upon an intricate compound rhythm into which several com- 

 ponents enter (see Barrell, "Rhythms and the Measurements of Geologic Time," 

 Bull. Geol. Sac. America, vol. xxviii, 1917), of which perhaps the chief is the 

 diastrophic deformation of the earth's crust. There may be a minor astro- 

 nomical component contributing to the rhythm of climatic change, but we 

 are as yet far from disentangling its effects from those of the more domi- 

 nant factors. 



I am. Sir, 



Yours truly, 

 April 16, 1920. G. W. Tyrrell. 



To THE Editor of "Science Progress" 



A GREAT DEFAULT 



From Brig.-Gen. E. H. HILLS, C.M.G., R.E., F.R.S. 



Sir, — The essay by Sir Ronald Ross in your April number, entitled ' A 

 Great Default,' exhibits in a clear light the very small amount of scientific 

 thought that our administrative services bring to their task of government. 



