THE STUDY OF THE ROCKS 



ing the fossils of these various rock formations, from 

 the earliest to the latest, can tell which animals and 

 plants have been longest upon the earth. Some of 

 the living forms of to-day have existed from very- 

 early times; we know, for example, that cock- 

 roaches were upon the earth long ages ago, for their 

 fossil remains are found in the very early rocks. 



For the most part the fossils of animal forms do 

 not make good objects for the microscope. The 

 Foraminiferaf minute creatures dwelling in shells, 

 are the most suitable for microscopic examination 

 and very beautiful some of them are. They are best 

 examined by reflected light, for then their little 

 shells show their delicate pearly sheen. 



Sponge spicules, as may be guessed from their 

 hardness, are common in the fossil state; there are 

 also fossils of sea-anemone ancestors and fossils also 

 of Hydra colonies. The last named, known as 

 graptolites, look not unlike lead-pencil marks on the 

 rock. They exist in many forms ; Diplograptus has 

 a stem with two rows of cups in which the little 

 creatures lived long ages ago; Monograptus has 

 but one row of cups upon its stem, whilst Didymo' 

 graptus is a branched form. These fossils are by 

 no means rare and are worth studying under the 

 microscope. 



There are curious fossils knows as Trilobites, not 

 unlike the present-day wood lice; they too may be 

 studied, for some of them show all their character- 

 istic markings as plainly as they must have appeared 

 on the living animals. 



135 



