%^ PHOSPHATIC CHALK. 285 



papers on the subject (2), we are told that of all the deposits yet 

 discovered, even those with a cubic capacity of more than a million 

 metres, every one has been proved to be lenticular in form. The 

 greatest length of these lenticular beds rarely exceeds one kilometre 

 (1,100 yards), and their breadth two or three hundred metres, while 

 in thickness the phosphatic rock ranges from nothing at the edges 

 of the lenticle to upwards of twenty metres in its central portions. 

 This, then, is the probable explanation of the fact that even at 

 Taplow the deposit has not been recognised beyond the pit and its 

 immediate surroundings. It is, doubtless, of a lenticular form, and 

 probably of no great extent. The other pits and sections in the 

 neighbourhood, which are neither numerous nor conveniently situated, 

 seem in the few cases where they expose the right zone in the Chalk 

 to lie beyond the limits of the lenticle. What these limits are, and 

 in what directions the lenticle extends can only be determined by 

 borings or by tunnelling from the known outcrop, either of which 

 operations would entail a greater expense than would be repaid by 

 the working of the deposit in the present state of the phosphate 

 market. 



The study of a rock, however, by geologists is little influenced by 

 its price per ton, and any new discoveries of this interesting variation 

 of the ordinary type of Chalk will receive due attention. But where 

 the next find will be made becomes more uncertain as time passes. 

 The Belgian phosphate occurs at a higher zone in the Chalk than is 

 known to exist in this country. The French deposits, on the other 

 hand, though not strictly on one horizon, lie high up in the Upper 

 Chalk of English geologists, and at or near the horizon of the Taplow 

 Chalk. In our locality this horizon lies close beneath the Tertiary 

 beds, but in other districts, and even in the immediate neighbourhood 

 of Taplow, these strata rest upon other parts of the Upper Chalk, 

 either higher or lower than that in which the phosphatic beds are 

 situated. For the country generally, therefore, we require a surer 

 guide than that afforded by the Tertiary boundary. 



The Chalk at first sight presents so great a sameness from top 

 to bottom that an unpractised observer would have little chance of 

 distinguishing one portion of it from another. It can, however, be 

 divided by lithological characters and fossil-contents into three 

 generally well-marked subdivisions, known as the Upper, Middle, 

 and Lower Chalk. The lower limit of the Upper Chalk is drawn at 

 a nodular bed, not only conspicuous from its hardness and other 

 peculiarities, but capable of being traced over a large part of the 

 country, and of being laid down with accuracy on the Geological 

 Survey maps ; which, in fact, has already been done over consider- 

 able areas. In the Chalk Rock, therefore, we shall have a definite 

 base-line from which to reckon upwards in a search for the zone 

 where phosphatic chalk may be expected to occur. Some further 

 assistance may be rendered by the existence of zones in the Upper 



