192 THE SECONDARY AGES. 



at work in accumulating those masses of chalk which now 

 constitute the cliffs and downs of southern England.* 

 There is nothing, we repeat, very difficult of explanation 

 in the mere sediments of the secondary ages, while, as 

 during all other periods, vulcanic eruptions were here and 

 there breaking up their continuity, and occasionally alter- 

 nating with their strata. In England the secondary rocks 

 have suffered little or no disturbance from igneous agency, 

 and hence their broad and unbroken succession in that 

 country. All to the south-east of a line roughly drawn 

 from the Severn to the Tees is occupied by these forma- 

 tions, and the reader has only to cast his eye over the 

 geological map, to 'perceive how regularly and continuously 

 they follow each other. In Scotland and Ireland, however 

 (Skye, Giant's Causeway, &c.), as well as in the Jura and 

 other Continental districts, the secondary rocks are much 

 disturbed and altered by igneous eruptions a proof that 

 during this, as during all other ages, vulcanicity displayed 

 itself only along certain lines and within certain centres 

 with notable intensity. 



As a life-period, the secondary systems, though charac- 

 terised each by its own peculiar forms, have yet so many 

 features in common, that they may conveniently be re- 

 garded as representing one great and unbroken cycle of 

 world-history. As marine deposits they abound in all the 

 lower forms of life foraminifera, sponges, corals, encrinites, 

 star-fishes, sea-urchins, polyzoa, shell- fish, Crustacea, and 

 worms ; contain at the same time many orders of insects ; 

 are replete with numerous families of fishes and reptiles ; 

 and now, for the first time in geological history, give 

 unmistakable evidence of birds and mammalia. Without 



* For more detailed explanation of this deep-sea calcareous ooze or 

 mud, see page 44, in the sketch entitled " Waste and Eeconstruction." 



