O'lS EVENING DISCOURSES. 



responsible for the conquest of England. Ilalley, who was delighted that au 

 Englishman should be first to recognise the periodical character of the comet, 

 ■would no doubt have been deeply interested by this curious association with an 

 important epoch in our history had he been led to suspect it. 



The verification of these dates, rendered probable by Hind, has been nobly 

 carried out by the Englishmen above mentioned, Messrs. Cowell and Crommelin. 

 (Mr. Crommelin is an Irishman, but an assembly of the British Association in 

 Dublin takes no note of such details.) Mr. Crommelin wrote to the present 

 ecturer on July 28 : — 



' We have now carried back Halley's Comet to B.C. 87 (August) with certainty 

 (oue revolution earlier than Hind's list), and with fair probability to B.C. 240 (May). 

 Before this observations are completely wanting. Hind is one and a half vears 

 too late for his 603 (a.d.) return (it really was C07 March), but all his earlier 

 returns are right up to the beginning of his list ( — 11= B.C. 12). We find 

 1910 April 12-9 for the next passage, but are going over the work again by a new 

 method.' 



This date makes the comet at its brightest, to our earthly view, in May 1910. 

 In that month there will be a total eclipse of the sun, visible in Tasmania, and the 

 most glorious view of the comet obtainable at this return will probably be that 

 accorded to those in Tasmania during totality. (Does this signify an extension of 

 the national associations of the comet to our colonies?) 



The year 19l0 is also the tercent(!nary of the first use of the telescope by 

 Galileo. We are reminded how much we owe to astronomical work in the three 

 centuries since elapsed. Not merely are wo no longer terrified by comets ; our 

 whole conception of the majmitude aud meaning of the universe has been changed. 

 Much of what we have gained we owe to H alley, who showed that comets were 

 no strange monsters, but members of our family (solar) circle ; and, far more than 

 this, elicited the ' Principia' from Newton. When we see Halley's Comet let us 

 think with reverence of this great Englishman and his work. 



MONDA Y, SEPTEMBER 



The Colorado Canyon. By Professor W. M. Davis. 



The Grand Canyon of the Colorado, in the plateau of Northern Arizona, is not 

 only impressive as an exhibition of normal erosion in a dry climate, but is even 

 more instructive in presenting a natural geological section in its walls from which 

 one may learn of several periods in the past, during each of which the amount of 

 erosion accomplished was vastly greater than that required in the excavation of 

 the canyon. The plateau is built of a heavy series of horizontal palaeozoic strata 

 which rest partly on a body of crystalline schists, partly on a series of east-dippiug 

 strata. If one imagine the horizontal strata to be" removed, an even surface 

 truncating the crystaliine rocks and the inclined series is seen. This is evidently 

 the result of widespread erosion by Avhich the former upward extension of the 

 inclined series aud its crystalline foundation were reduced from their original 

 extension. Restoring the lost mass and imagining it turned so as to place the 

 inclined series in a horizontal position, these strata may be stripped off, revealing 

 the remarkably even fioor of their crystalline foundation. This floor is also 

 evidently the work of erosion of a formerly greater crystalline mass which may 

 well have once had a mountainous height above what now remains. Further 

 backward in time the history of the rock structures has not yet been traced. The 

 reduction of the ancient crystalline mountains to an even surface must have 

 required a vastly longer time than the erosion of the Grand Canyon, for the erosion 

 of the canyon represents only the good beginning of a cycle of "erosion, while the 

 reduction of the ancient mountains to a plain represents the essential completion 

 of a cycle of erosion. The deposition of a heavy series of strata, some 10,000 feet 

 thick, upon the crystalline floor represents another period of time vastly longer 



