72 HANDBOOK OF FOSSILS. 



closely matted together and partially mineralized. Vegetable 

 matter in this form is known as lignite, and is one of the first 

 stages towards the formation of coal out of plant remains. Below 

 this lignite band we find our leaf-bed getting sandier and 

 sandier, and losing all trace of the plants by degrees till it becomes 

 almost pure sand. Here and there, however, it contains some 

 curiously shaped masses, which, when broken through with the 

 hammer, seem composed of nothing but the same grains of sand 

 cemented together into a hard mass. In one there is, however, a 

 curiously shaped hollow, which, upon examining it closely, you 

 will see is a perfect cast of a small shell that has itself disappeared. 

 A drop of acid on it fizzes away and sinks in between the grains 

 of sand which in this spot become loose. A mass of sand or 

 particles of clay thus cemented together, be it by iron, lime, or 

 any other substance, is termed a "nodule" or "concretion" 

 and in this particular instance has been formed as follows : The 

 rain-water falling on the sand where it comes to the surface sinks 

 in and filters through the bed. Now there is always a certain 

 amount of carbonic acid in rain-water, and this acid acted on 

 the carbonate of lime of which the shell was composed, dis- 

 solving and dispersing it amongst the neighbouring grains of 

 sand where it was re-deposited, cementing them together as we 

 have seen. The bottom of this bed of sand we find to be just 

 fifteen feet from the lignite band when measured at right-angles 

 to the bed, and it is succeeded by a hard greyish rock, which 

 requires a smart blow of the hammer to break it, but the surface 

 of which, where it has been exposed to the weather, is much 

 crumbled (" weathered"}, and breaks readily into small pieces. 

 It is easily scratched with the point of a knife, and therefore is 

 not flint ; moreover, it fizzes strongly when touched with acid 

 hence there is a great deal of carbonate of lime in it, and we 

 know that it is limestone. 



Limestones are very largely, sometimes almost entirely, made 

 up of the calcareous portions of marine creatures, such as the hard 

 parts of corals, the tests of sea-urchins, the shells of mollusca, 

 etc., welded, so to speak, into one mass by the heat, pressure, and 

 chemical changes which the bed has undergone since its depo- 

 sition at the bottom of the sea. There would be every reason, 

 therefore, one might suppose, to expect a number of fossils in 

 this bed ; but, alas ! disappointment awaits the young explorer, for 

 with the exception of chalk and a few other limestones, these 

 rocks are generally of such uniform texture that on being struck 

 with the hammer they split through fossils and all, the fractured 

 surface only too frequently showing nought save a few obscure 



