26 GEOLOGICAL HISTORY. [CH. 



which in part consists of millions of ininute shells of Fora- 

 minifera, in part of broken fragments of larger shells of extinct 

 molluscs, and to some extent of the remains of siliceous sponges. 

 As a general rule, limestones and chalk rocks are ancient sedi- 

 ments, formed in clear and comparatively deep water, composed 

 in the main of carbonate of lime, in some cases with a certain 

 amount of carbonate of magnesium, and occasionally with a 

 considerable admixture of silica. 



In such rocks land-plants must necessarily be rare. There 

 are, however, limestones which wholly or in part owe their 

 formation to masses of calcareous algae, which grew in the 

 form of submarine banks or on coral reefs. Occasionally the 

 remains of these algae are clearly preserved, but frequently 

 all signs of plant structure have been completely obliterated. 

 Again, there occur limestone rocks formed by chemical means, 

 and in a manner similar to that in which beds of travertine are 

 now being accumulated. 



Granites, basalts, volcanic lavas, tuffs, and other igneous 

 rocks need not claim our attention, except in such cases as 

 permit of plant remains being found in association with these 

 materials. Showers of ashes blown from a volcano, may fall 

 on the surface of a lake or sea and become mixed with sand 

 and mud of subaerial origin. Streams of lava occasionally 

 flow into water, or they may be poured from submarine vents, 

 and so spread out on the ocean bed with strata of sand or clay. 



Passing from the nature and mode of origin of the 

 sedimentary strata to the manner of their arrangement in 

 the Earth's crust, we must endeavour to sketch in the merest 

 outline the methods of stratigraphical geology. The surface 

 of the Earth in some places stands out in the form of bare 

 masses of rock, roughly hewn or finely carved by Nature's 

 tools of frost, rain or running water ; in other places we have 

 gently undulating ground with beds of rock exposed to view 

 here and there, but for the most part covered with loose 

 material such as gravel, sands, boulder clay and surface soil. 



In the flat lands of the fen districts, the peat beds and 

 low-lying salt marshes form the surface features, and are the 

 connecting links between the rock-building now in progress and 



