1 94 GEOLOGY. 



we can see from our reading of the life-record of the 

 rocks, was adapted to the circumstances of the case. 

 Though all the four grand divisions of animals were rep- 

 resented, there were few vertebrates or articulates, and 

 these were of the lower orders, because the circumstan- 

 ces of the world were not suited to those forms of life. 

 The animals were mostly marine. There were no air- 

 breathing animals, because the atmosphere was not in a 

 fit state for them. There was too much carbonic acid in 

 it, of which the air was to be relieved in after ages, as 

 will be shown you in another chapter. There were no 

 fishes, partly, at least, because they need dissolved in the 

 water which passes through their gills air that has a 

 good proportion of oxygen in it (Part II., 93). In re- 

 gard to the adaptation manifest in that age of the earth, 

 Agassiz very forcibly remarks : " Let us remember, then, 

 that in the Silurian period, the world, so far as it was 

 raised above the ocean, was a beach, and let us seek there 

 for such creatures as God has made to live on sea-shores, 

 and not belittle the creative work, or say that he first 

 scattered the seeds of life in meagre or stinted measure, 

 because we do not find air-breathing animals when there 

 was no fitting atmosphere to feed their lungs, insects 

 with no terrestrial plants to live upon, reptiles without 

 marshes, birds without trees, cattle without grass all 

 things, in short, without the essential conditions for their 

 existence." 



283. Carbonaceous Matter in Silurian Shales. There 

 was no coal formed in this age, for there was no vegeta- 

 ble growth sufficient for this purpose ; but in the shales 

 there is a considerable amount of carbonaceous matter 

 in some localities, even, in some cases, up to 20 per cent. 

 This came mostly from the sea-weeds, which, gathering 

 the carbonic acid from the air, appropriated to their own 

 growth the carbon, and gave the oxygen to the animals 

 of the sea (Part II., 95). 



284. Hydrozoa and Bryozoa. In Fig. 102 we have the 



