on the Sand-pipes in the Chalk near Nonoich. 457 



for every chemist knows, that during the process of drying 

 precipitates of this nature, there are always fissures produced 

 in the precipitated substance as the fluid evaporates or fillers 

 away. Such effects must have been a necessary consequence 

 in the chalk as it approximated to dryness, and hence the 

 primary cause of the cavities we are investigating. The dif- 

 ference of size in the cavities is, I conceive, a consequence 

 that would inevitably result, not only from some parts of a 

 mass being more exposed to the sun than others, but also to 

 various circumstances which would cause partial drying of the 

 chalk; and above all to the failure of support from the under 

 strata, from subterraneous springs, &c. Indeed there seems 

 reason to believe that the largest sand-pipes have been formed 

 by the complete division of a mass of chalk from the bottom 

 to tlie top arising from the above-named causes. 



Mr. Lyell further observes that " Large unrounded no- 

 dules of flints, found in the sand-pipes, still preserve the ori- 

 ginal form and white coating." And in the smaller pipes 

 *' horizontal layers of siliceous nodules still remain m situ, 

 not having been removed, together with the chalk in which 

 they must have been originally imbedded." 



If these cavities were formed by the action of " water 

 charged with acid," as Mr. Lyell supposes, how is it that it 

 did not dissolve the chalk from the surface of those flints " m 

 situ," upon which its action must have been as great as upon 

 any other part of the surface ? They remain, however, still 

 with the chalk adhering to them, as pure and as perfect as 

 that in any other part of the mass. 



Mr. Lyell goes on to state, " It is clear from the manner 

 in which the large detached flints are dispersed through the 

 contents of the wide sand-pipes, that excavation and filling of 

 the pipes were gradual and contemporaneous processes. For 

 had the tubes, some of which are from 50 to 60 feet deep, and 

 seven yards or more wide, been hollowed out of the chalk 

 before the introduction of any foreign matter from above, 

 great heaps of unrounded flints must have fallen to the bot- 

 tom, derived from all those intersected layers of flint which 

 formed part of the chalk above." 



There is no doubt but they have done so, and I do not see 

 that it follows, on that account, that excavation and filling up 

 were contemporaneous processes ; for we have no data to 

 suppose that what is now the bottom of a wide and deep sand- 

 pipe was so originally: such an immense division in masses 

 of chalk must have been partially filled up by the sides of the 

 hollows being washed down to the bottom of the cavity, and 

 most probably have covered over the flints which fell down it 



