232 



On the Silica Strata of the Lower Chalk, 



Our attention was first called to the examination of this bed 

 of rock, as it formed externally an easily distinguishable member 

 of the chalk-marl series," and was consequently selected for 

 investigation. Its peculiar physical properties, viz. its friability, 

 its low specific gravity, its porosity, and consequent mechanically 

 absorptive power, were very striking, especially as it has been 

 very largely applied to the covering up of dung-heaps during 

 several years, besides having been extensively used as a direct 

 manure with signal benefit on the upper chalk soils, as well as 

 on some very sandy lands. For this purpose it has been carted 

 to distances of from two to three miles, both to the north and 

 south of the quarry. In addition to these circumstances we 

 had long known the outcroppings of the " malm " to be naturally 

 (that is, when not obscured by want of drainage or by other bad 

 cultivation) of the most fertile character, constituting, as we 

 have stated in our former papers, some of the very best hop and 

 wheat lands in the kingdom. 



On the south-west of the parish of Farnham there is a large 

 development of the gault and upper greensand in juxta-position, 

 forming a succession of outcrops near the surface, which is 

 chiefly comprised in Mr. Paine's farms called " Dippen Hall " 

 and " Deans " in the Ordnance maps. The whole stratum 

 between the gault and the grey or dirty-white marl (No. 7, 

 vol. xii.) is about 100 feet thick. It invariably assumes a rocky 

 appearance on receding from the gault, getting rid more and more 

 of its argillaceous character till the rock stands out distinctly, 

 though still of a very soft friable nature, crumbling into powder 

 on exposure to the weather. This is the portion that has been 

 quarried for manure. It contains from 25 to 30 per cent, of 

 soluble silica, and occupies a central position in the whole 

 stratum. The range between these two percentages is from 30 

 to 40 feet in vertical depth. Ascending from this middle portion 

 of the stratum it gradually assumes a more compact form of 

 rock, which contains from 30 to 70 per cent, of soluble silica. 

 There are, however, intermediate layers of softer earth, which 

 we find have invariably a lower percentage of silica, as, for ex- 

 ample, No. 23, in Section lit., between 24 and 25 ; and No. 72, 

 between 71 and 73, in Section I. We now approach the true 

 firestone, in some cases with, in others without, carbonate of 

 lime. Here, however, it differs from the firestone of Merstham 

 and the Isle of Wight, in having none of the green grains which 

 are so thickly interspersed in these districts. There is usually 

 one — oftentimes two beds of the blue limestone running through 

 the firestone. This is a most excellent building-stone. The 

 highest percentage of soluble silica yet obtained is from a layer 

 about 10 feet thick and 15 feet below the firestone: here from 

 70 to 75 per cent, may be procured. From this culminating 



