Jan. 5, 1888] 



NATURE 



22 



on the materials that are swept along by them, by which grains 

 of one size and weight are laid down at one time, and of another 

 size and weight at another, Changes in the nature of the 

 material in suspension also occur through which the deposit 

 may be at different times more siliceous, argillaceous, or cal- 

 careous. This is doubtless in most cases a true explanation of 

 the cause of lamination in rocks, but it is not a full one, nor 

 does it account for stratification such as I am about to describe. 

 In sand dunes composed entirely of siliceous grains such as 

 are seen on the west coast of Lancashire between Liverpool 

 and Southport-, a strong false-bedded lamination is often beauti- 

 fully developed. This is best seen when the sand-hills are moist 

 from recent rainfall, and the talus has been cut away by a high 

 tide, leaving a vertical face of sand to the shore side. After 

 this has occurred a gentle wind will weather out the structure of 

 the sand-hill in a remarkable manner. The layers often stand 

 out several inches in projecting mouldings and fillets, while the 

 finer laminae are wonderfully developed. I have often minutely 



Fig. I. — View of sand-dune, showing the bedding and laminae weathered out 

 by denudation, a, shore ; b, loose talus ; c, vertical cliff of sand ; 

 d, surface of sand-dune. 



examined the constitution of these beds, but have been unable 

 to detect any difference in the sizes of the constituent grains of 

 the several beds or laminae. What makes the fact more striking 

 is that the grains are generally and in many cases much rounded. 

 An examination, however, shows that the laminae projecting from 

 the face of the sand-cliff are much harder and more solid than 

 the portions between them that have weathered back. They 

 can, in fact, be broken off in pieces by the fingers without 

 crumbling. The, grains of sand, I must observe, are only 

 temporarily bound together by the capillary attraction of the 

 water. 



The explanation which suggests itself to me is that the grains 

 of sand, according to the state of the weather during deposi- 

 tion, are at one time more completely aggregated than at 

 another. The shore sand, I have noticed, is greatly affected 

 by the state of the water it is laid down by. In one place may 

 often be seen a stretch of hard fine sand, while in another at the 

 same level the sand may be soft, both being at the same point 

 of saturation. It is well known to builders that pouring water 

 on loose sand tends to solidify it, therefore it is most probable 

 that the state of the weather influences the solidity of aggrega- 

 tion of the surface of the sand dunes and assists to build up layers 

 of different density. 



Between the projecting fillets already described as weathered 

 out of the sand cliffs the sand is looser and more porous, and, 

 drying faster, falls away from the face at a greater rate than the 

 compacted beds. In sand heaped together by the wind there 

 are few, I should think, would a priori look for much internal 

 structure ; yet here are the most undoubted evidences to the 

 contrary which are generally passed by, being looked upon as a 

 matter of course not demanding further thought ! If we con- 

 sider in what way the constituent grains naturally arrange them- 

 selves by gravity, we shall, I think, get an additional c!ue to the 

 cause of lamination. Even if the grains were as round as shot 

 they would by gravitation tend to arrange themselves in parallel 



planes, the upper grains falling into the interstices of those 

 next below them, so — 



Fig. 2. 



If, on the contrary, the grains have a long and] short axis, they 

 will tend to lie with the longer axis parallel to the plane of 

 deposition, so — 



Fig. 3. 



With irregular fragments the arrangement will not be so perfect, 

 but they also will tend to be laid down in definite planes. 



An examination of specimens of laminated sandstone shows 

 that a fracture vertical to the plane of lamination exhibits a 

 more jagged surface than a fracture parallel to the plane of 

 lamination. This it is that gives the strength to sandstone to 

 resist transverse stress. 



It is thus seen that nature adopts the same principle to build 

 up sandstone that a mason does to build a wall. From the way 

 in which the particles arrange themselves a natural "bond" is 

 produced. The grains " break joint," as it is technically called — 

 that is, the joints are not vertically over each other— while the 

 planes of deposition correspond to the "courses" of a wall. 

 The principle here explained I have seen well exhibited in con- 

 glomerates formed of flattish oval pebbles. The mode of ag- 

 gregation of the particles of a sedimentary rock, due to the 

 ordinary dynamical laws governing deposition, and independent 

 of the coarseness or fineness of the grains of successive layers, is 

 an important factor in its constitution, which seems hitherto not 

 to have attracted much attention. T. Mellard Reade. 



Total Solar Eclipse of October 29, 878. 



In Nature for March 11, 1875, vol. xi. p. 365, a computa- 

 tion is given of this eclipse, based on an entry in the " Annales 

 Fuldenses," which runs thus : " Sol quoque in 4 kal. Novembris 

 post horam nonam ita obscuratus est per dimidiam horam, ut stelJae 

 in coelo apparent, et omnes noctem sibi imminere putarent." 



The computer found that the sun rose on that day at P^ulda at 

 7h. 12m. apparent time, 6h. 57m. mean time, that the partial 

 phase began at oh. 56m., and ended at 3h. 24m., totality com- 

 mencing at Fulda at 2h. grr. 32s. local mean time, and continu- 

 ing im. 41S. till 2h. iim. 13s. He seems to have been puzzled, 

 however, by the statement of the annalist that the darkness 

 occurred " post horam nonam," observing plausibly enough that 

 the ninth hour from sunrise would be 4 p.m. 



It is shown, however, in Dr. Smith's "Dictionary of Christian 

 Antiquities," vol. i. p. 793, that the day then employed by the 

 Church was the natural day extending from sunrise to sunset, 

 which was conceived to be divided into twelve hours (shorter of 

 course in winter than in summer) ; so that the first hour was the 

 twelfth part of the natural day, which began with sunrise ; the 

 sixth hour that which ended when the sun crossed the meridian, 

 and so on. 



The question, then, which arises is this : At what point of local 

 mean time did the ninth natural hour end at Fulda on October 

 29, 878 ? 



The sun rose at 7h. 12m. apparent time : this would give a 

 semi-diurnal arc of 4h. 48m., or 9h. 36m., as the duration of the 

 natural day, one-twelfth part of which, or 48 minutes, would be 

 the length of the natural hour. As nine such hours would con- 

 tain 432 minutes, it is clear that the ninth hour after 6h. 57m. 

 the local mean time of sunrise would end at 2h. 9m., and the 

 half-hour of darkness mentioned would have extended from 

 2h. 9m. to 2h. 33m. As the computer reckoned that totality 

 lasted from 2h. 9m. 32s. to 2h. iim. 13s., the obscuration would 

 have been gradually passing away during that period. 



The coincidence between the record and the calculation is a 

 very striking one, and testifies at the same time to the veracity 



