PRELIMINARY CONSIDERATIONS. 



13 



alive, or formed part of living organisms. This is the case 

 with some limestones, which consist of microscopic shells, or 

 of larger shells, corals, and similar calcareous organisms, either 

 entire or broken into fragments and cemented together with 

 pasty or crystalline limestone filling their interstices. This 

 may be seen in Fig. 9, which represents a magnified slice of a 

 Silurian limestone. Coal in like manner consists of carbonised 







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Fig. 9.— Section of Trenton limestone, magnified, showing that it is composed of 

 fragments of corals, crinoids and shells. Montreal. 



vegetable matter, retaining more or less perfectly its organic 

 structure, and sometimes even the external forms of its consti- 

 tuent parts. More frequently, fossils are dispersed more or less 

 sparsely through the substance of beds composed of earthy 

 matter ; and they have usually been more or less affected by 

 chemical changes, or by mechanical pressure, or are mineral- 

 ised by different substances which have either filled their pores 



