198 



KNOWLEDGE, 



[October 1, 1892. 



flint gravels of the valleys of the south of England, as 

 well as the beashes of our southern coasts, and the 

 numerous tertiary deposits composed of flint pebbles, 

 remain to us as silent witnesses of the vast denudation of 

 the upper chalk which has taken place in this country. 

 Eemembering that the proportion of flint to chalk is only 

 from four to six per cent., and also bearing iu mind that all 

 the flints in our gravels have been considerably reduced in 

 size by the action of water, we may fairly say that every 

 cubic yard of pure flint gravel represents the removal of 

 at the very least twenty cubic yards of pure chalk ; and to 

 this we have to add all the lower chalk which has been 

 denuded without leaving any solid residue. Moreover, 

 when we recollect that this denuding jirocess has been 

 going on ever since the eocene period, and that our river 

 gravels only represent a small portion of the flints left by 

 the denudation of the chalk during the latter jmrt of this 

 protracted period of time, we may gain some faint concep- 

 tion of how enormous this denudation must have been. 



Although the flint gravels of our rivers afl'ord some 

 estimate, however faint, of the denudation of the chalk 

 during the pleistocene period, it would be quite incorrect 

 to assume that the flint pebbles forming the beaches of 

 om- southern coasts present a record of the amount of 

 denudation which has taken place during the modern 

 period. We have already mentioned that a freshly-broken 

 flint presents a uniform blackish-blue colour throughout 

 its interior, and any flint pebble on the seashore which had 

 been recently derived from its native chalk would, when 

 broken, present a similar appearance. As a matter of 

 fact we shall find, however, that at least ninety per cent, 

 of such pebbles are stained yellow, brown, red, or black 

 internally, and as most of the flint fragments in many of 

 our older gravels are likewise similarly stained, we shall 

 have little hesitation in coming to the conclusion that our 

 modern sea-beaches are largely derived from the breaking 

 up of such old gravel beds, and the subsequent rounding of 

 these irregular fragments of flint into pebbles by the action 

 of the sea. The staining of the flints is of course due to 

 the large amount of ferruginous matter contained in the 

 gravels, and owing to the banded nature of the original 

 flint it frequently gives rise to an agate-like appearance in 

 the pebbles. Many of the pebbles in our beaches are, 

 however, derived from stiU older sea-beaches, like the one 

 now remaining at the southern extremity of the Isle of 

 Portland, while others, again, owe their origin to the 

 breaking up of the eocene Woolwich and Reading beds, 

 which are largely composed of flint pebbles. Sometimes, 

 indeed, fragments of those old beds also occur iu the river 

 gravels, where blocks of the Hertfordshire conglomerate — 

 the equivalent of the Woolwich and Reading beds — may be 

 met with. 



We have thus abundant evidence of the exceeding in- 

 destructibility of fliat, and how it may go on from one 

 formation to another to tell, when rightly interpreted, the 

 various steps in the denudation of our country. 



In addition to being frequently stained internally, the 

 observer will also not fail to notice that all of the flints 

 in our river gravels have acquired a white or yellow porce- 

 laneous external coating, quite diflerent from the interior ; 

 and it is beheved by Sir .John Evans that this white 

 coating has been produced by the removal of a portion of 

 the flint which was still soluble " by the passage of in- 

 filtrating water through the body of the flint." That such 

 a process must have been of inconceivable slowness, and 

 must have required countless years for its accomplishment, 

 goes without saying. We have, indeed, some inkling of 

 how extremely slow this process must be, by the circum- 

 stance that the fractured surfaces of the flints built into 



the walls of our very oldest churches show not the 

 slightest change from their pristine condition. When, 

 however, we examine the chipped flint implements of 

 our river gravels and caves, like the one shown in our 

 first illustration, we find their surfaces altered precisely iu 

 the same manner as the flint fragments by which they are 

 accompanied. Hence we gain, from a totally independent 

 source, some idea of the immense antiquity of the period 

 when the old palreoUthic hunters inhabited the south of 

 England. 



Having thus reached the subject of flint implements, 

 we feel tempted to enter into the consideration of some of 

 their diflerent types a,nd the beds in which they occur, but 

 editorial limitations of space forbid our wandering into 

 such entrancing paths. Even, however, without entering 

 into this part of the subject, we trust that what we have 

 written will serve to show that a " Flake of Flint," when 

 considered from all points of view, is to the full as in- 

 teresting as a " Lump of Chalk." 



THE FACE OF THE SKY FOR OCTOBER. 



By Hekbekt Sadler, F.R.A.S. 



THERE does not seem to be any visible diminution 

 in the number of spots and facuhiB on the solar 

 disc. There will be an eclipse of the Sun, visible 

 over a great part of North America, on the 20th, 

 y^y^jths of the disc being obscured. Conveniently 

 observable minima of Algol occur at lOh. 10m. p.m. on 

 the 11th, and Gh. 58m. p.m. on the 17th. The Nova Aurigfe 

 has made a brief reappearance, but appears to be sinking in 

 magnitude. 



Mercury is too near the Sun to be visible in October, 

 being in superior conjunction on the 8th. Venus is a 

 morning star, and is still well placed for observation, 

 although her brightness at the end of the month is only 

 about one-half of what it was at the beginning of June, 

 and her diameter is also decreasing notably. She rises on 

 the 1st at Ih. 4Gm. a.m., with a northern declination of 

 12° 50^', and an apparent diameter of 20.1", y'y'yths of the 

 disc being illuminated. On the 17th she rises at 2h. 19m. 

 A.M., with a northern declination of 7° 44', and an apparent 

 diameter of 18", iVij^'^s of the disc being illumaiated. 

 On the 31st she rises at 2h. 49m. a.m., with a northern 

 declination of 2° 10', and an apparent diameter of 16", 

 -j'jjths of the disc being illuminated. During the month 

 she passes through Leo into Virgo, being near llegulus on 

 the 5th. 



Mars is an evening star, and is better situated as to his 

 elevation above the horizon, though his apparent diameter 

 has markedly decreased, and, of course, his brightness 

 also — at the end of October being only a quarter of 

 what it was at opposition. He rises on the 1st at 4h. 10m. 

 P.M., with a southern declination of 20"^ 59', and an 

 apparent diameter of IG-U", the defect of illumination on 

 the following limb being very marked. On the 31st he 

 rises at 2h. 30m. p.m., with a southern declination of 15° 13', 

 and an apparent diameter of 12i", the gibbous form of the 

 planet being very apparent. During the month he is in 

 Capricornus, and makes an exceedingly near approach to 

 the third magnitude star 8 Capricorni on the 25th. Un- 

 fortunately, at the time of nearest appulso. Mars will be 

 below the horizon in England. 



Jupiter is a superb object in the evening sky, and, as we 

 remarked in the "Face of the Sky for September," is 

 visible to the naked eye in sunlight, the present opposition 

 of the planet being one of the very favourable ones which 

 occur once every twelve years. Jupiter rises on the 1st at 



