THEIR ROCK-SYSTEMS. 189 



igneous centres, few of the strata have suffered much meta- 

 morphism ; the areas and boundaries of the seas of deposit 

 are much more apparent ; and birds and mammals, unknown 

 in previous systems, are now met with in some abundance. 

 Of course, as the secondary ages embrace several systems 

 and formations, there will necessarily be considerable differ- 

 ence, both in rocks and fossils, between its respective series ; 

 but notwithstanding these differences there is still similarity 

 sufficient to enable us to treat them as a great group, or 

 rather as occupying a continuous and connected portion of 

 the earth's geological history. And after all, it is only by 

 groupings and generalisations of this kind that the non- 

 scientific reader can be expected to catch a glimpse of world- 

 history, the details and reasonings of which must be 

 worked out by the slow and patient research of the profes- 

 sional inquirer. 



The earliest of these Secondary systems, we have said, 

 is the Trias, a series of reddish-coloured sandstones, shelly 

 limestones, and saliferous or salt-yielding marls; the second, 

 the Oolite, a series of calcareous freestones, clays, and 

 shales, with occasional coals and ironstones ; and the third, 

 the Chalk, so notably composed in the south of England of 

 that white, soft, earthy limestone we call "jchalk," and its 

 underlying clays and greensands. With the exception of 

 a subordinate series of strata that lie between the Oolite and 

 Chalk, and known as the " Wealden " from their occurrence 

 in the wolds or woodlands of Kent and Sussex, all the 

 strata of these secondary formations are eminently marine, 

 and are charged throughout with marine organisms. The 

 Wealden sands, sandstones, and clays point more to estua- 

 rine conditions, and where they occur the coals and lignites 

 of the Oolite and Chalk indicate the existence of land-sur- 

 faces; but all the other strata the shelly and coralline lime- 



