the buildinxr of an island. 89 



2. The Relative Ages of the Rocks. 



While it is not possible to state the age of any geological formation in 

 definite time measures of any kind, it is possible to ascertain the order in which 

 the deposits have followed each other and thus to state their relative cigts. In 

 our own island we can see that the limestones and marls must be younger than 

 the indurated clays, because thev lie upon the clays ; and we can probably 

 extend this conclusion to apply throughout the east and west axis of the 

 West Indies; but how are we to get beyond that? In order to see how 

 geologists have been able to compare the ages of rocks in different countries 

 or different parts of the same country, it is necessary to consider a few facts 

 about fossils. 



Fossils. 



Fossils are the remains of living beings, whether animal or vegetable, 

 which have been preserved in the rocks. Generally it is the hard parts, such 

 as shell and corals, and the bones of animals, that have been preserved; but 

 occasionally the forms of the softer parts have been preserved likewise, the 

 forms of fishes, for example ; and insects, with forms and colours perfect, have 

 been kept for long ages embedded in tree gums, such as amber. The study 

 of the ancient Hfe revealed by fossils is known as Palasontology (meaning the 

 study of ancient beings), and it is one of vast extent. It shows us that, going 

 back in the history of the globe as recorded in the strata of its crust, we come 

 to a time when hardly anv of the species of plants and animals now living on 

 the earth had any existence, but the period had a life of its own; bevond that 

 again we come to another period, which, while containing some of the same 

 fossils as are found in the later period, is also marked by a special set of its 

 own ; and so on. But it may be asked how did the palaeontologists know 

 that thev were going hack in time? Suppose we ask a similar question about 

 our own island; how do we know that we are going baek in time when we pass 

 from the limestone formation to the clay formation ? Simply because we find 

 the clay formation belozu the limestones. The same reasoning applies to the 

 more extended study. Now it happens that England has afforded a particularly 

 good field for the investigation, for in that country the strata, though by no 

 means free from disturbances, yet lie over each other in a regular manner, the 

 lowest (oldest) being in the west and the others lying over them in succession 

 till we arrive at the highest (youngest) in the east. This succession has been 

 thoroughly studied by the English geologists, who have arranged the rocks in 

 systems, some of which have been named from the most characteristic of the 

 strata included in them, others from the parts of the world where they are 

 conspicuous, and others again (The Tertiary division) from the relative 

 abundance of their fossil remains belonging to species still living, These 

 systems are grouped into three divisions, the oldest rocks being classed 

 as Primary, the next as Secondary and the youngest set as Tertiary. 

 Some geologists add Quaternary, to include very recent rocks and others 



