1884.] 



MICROSCOPICAL JOURNAL. 



153 



intrusions of chalk flint. The main 

 fact is the general quartzosc character 

 of the whole. 



Next in order comes our familiar 

 gravel, with its sand layers of that 

 deep ferruginous hue, prized for our 

 garden walks. Of this I took sam- 

 ples from a section on Epping Forest, 

 near Loughton, made for a supply of 

 fine gravel. Here, one would have 

 thought, if anywhere, being in the 

 midst of a chalk flint debris, rolled 

 together to all sizes, that a vein of sand 

 must show the same form of silex. 

 My specimen was taken four feet from 

 the capping of loam, firmly compac- 

 ted together, and of a deep rusty color. 

 On submitting a slide made from this 

 to the microscope by polarized light, 

 I was astonished to find it so uncom- 

 promising of quartz. There were 

 other substances, yet extremely few 

 in number, and I am not able to pro- 

 nounce upon them, but not the smallest 

 atom of our familiar flint of which 

 every pebble around was composed. 

 But not satisfied with one specimen 

 of the sand, I took another from a 

 vein of a pale gray tint close by, and 

 the same results ensued, as indeed one 

 might have expected, only in such in- 

 vestigations one should never assume 

 anything, but resort to experiment. 



We now come to the series of the 

 Bagshot sands, to which I have allud- 

 ed, and testing a specimen from 

 Hampstead Heath, after washing it, 

 quartz is found to be the largest basis 

 of the deposit. Other particles, how- 

 ever, are seen in it, some of which 

 look like amber, and some fragments 

 remind one of the color of the Cairn- 

 gorm, yet it requires a mineralogist 

 to pronounce upon them. The char- 

 acter is also special in the presence 

 of dark specks and nearly black bodies, 

 and we must certainly seek in another 

 direction than flint shingle, of which 

 few signs are to be seen, as the factor 

 of the Bagshot sands. 



The vast mass of the London clay, 

 that deposit of estuary mud of a tropi- 

 cal sea, has its layers of sand repre- 

 sented at White Cliff and Alum Bays, 



Isle of Wight, and of these I have ex- 

 amined several specimens, represent- 

 ing upper, middle, and lower beds, 

 as well as other series, but they all 

 declare the same general facts, quartz- 

 ose sand, with few intrusions of flint, 

 more or less comminuted. So we 

 will take into consideration the layer 

 styled ' Pebble bed,' where the flint 

 is rolled into marbles of various sizes, 

 intermingled wath and embedded in 

 sand. Here, if anywhere, one would 

 expect to meet with atoms of flint in 

 abundance. But the examination of 

 the fine sand of the bed referred to 

 shows the same result, and it suggests 

 to us that flint abrasion produces very 

 small and thin flakes which easily 

 break up and disappear into very 

 minute parts, but that the harder 

 quartz, never taking the same form of 

 fracture, is a more enduring form of 

 silex. So that the one disappears 

 rapidly, whilst the other continues 

 an almost indefinite time. This is 

 the only way I can account for a phe- 

 nomenon so apparently singular ; but 

 I am open to the conviction of a better 

 solution if that can be given. The 

 result, then, is remarkable in the all 

 but absent flint particles. These are, 

 indeed, represented, but they are few 

 in number in comparison with those 

 of the quartz. There are other sub- 

 stances than this, but, as before, it is 

 the predominant form, although in 

 the midst of rolled flints. The sand 

 is very fine in character, the particles 

 of quartz very small, resembling those 

 of the brick clay. 



We now come to the chalk from 

 whose upper series our vast deposits 

 of flint shingle must have been de- 

 rived by the extensive denudation 

 and destruction to which it has been 

 subjected. Belonging, geologically 

 speaking, to this series are two layers 

 entitled ' the Upper and Lower Green- 

 sand,' separatee! from each other by 

 the Gault Clay. The Upper Green- 

 sand is best known to us by the lime- 

 stone, called familiarly clunch, fire- 

 stone, hearthstone, used extensively 



