THE PREPARATION OF THE EARTH FOR MAn's ABODE. 91 



outbursts added igneous rocks to the sedimentary deposits, 

 and so contributed to the picturesque beauty and varied 

 elevation of many locahties at the present time. Many of 

 the hills of the south of Scotland, including the well-known 

 Arthur's Seat and the Castle Rock at Edinburgh, owe their 

 existence to the eruptions of volcanoes in Carboniferous 

 times, as does the marvellous scenic beauty of the 

 Cumberland Lake District to volcanic action in Lower 

 Silurian times. 



It should be added that the Palaeozoic rocks as a whole 

 form a vast store-house, as it were, of economic minerals 

 available for man's use at the present time. Amongst the 

 treasures which they contain, in addition to those peculiar 

 to the coal-measures, are gold, silver, copper, mercury, 

 lead, platinum, zinc, and antimony. These ancient rocks, 

 too, by their general hardness and consequent greater 

 resistance to denuding agencies, have added great beauty 

 to the world, and so have in this way also contributed mucli 

 to the happiness of mankind. The most picturesque and 

 beautiful seenery is in regions where the Palaeozoic rocks 

 occur, forming as they do mountains and hills, deep valleys, 

 lakes, and irregular coast-lines with lotty and precipitous 

 chfFs. 



The Secondary rocks continue the record of change and 

 progress. Very many of the life-forms' of the older rocks 

 are absent, and thus tell us of the extinction of a great 

 number of species and genera and of some large groups. 

 The most noteworthy is the extinction of the entire group 

 of trilobites, which were most abundant in Silurian times. 

 These very well defined crustaceans ranged in time from 

 the Lower Cambrian to the Carboniferous epoch. 



Although the oldest of the three great divisions of the 

 Secondary rocks, the Triassic, is not, as a whole, well 

 adapted for containing organic remains, since the Triassic 

 rocks are largely formed of sandstones, yet one series of 

 strata, the Rhastic, has been sufticiently preservative to yield 

 decided evidences of a striking and most important advance 

 in animal organization ; for in these beds are remains ot 

 an animal in the highest zoological class, Mammalia. The 

 Microlestes antiquus was a small animal of one of the lowest 

 orders of the class, Monotremata, yet it was unmistakably 

 a mammal, and thus the highest class of the animal kingdom 

 is found to have been represented in early Secondary times. 



In the varied and highly fossiliferous Jm'assic formations, 



