A COMPARATIVE SEQUENCE 



formed of crystalline rocks (gneisses and schists), which occur 

 in the former as level plains, and in the latter as hog's-back- 

 like ridges, composed of the harder parts of the series. The 

 Rangatan and the volcanic chain are both volcanic, one being 

 formed of sheets of lava due to " plateau eruptions," and the 

 other of piles of ashes and lava as in ordinary craters. 



The floor of the Rift Valley is occupied by more varied 

 materials than either of the other zones, for we find upon it 

 ancient and modern lavas of various ages, the alluvium of 

 dried lake basins, recent river gravels, and deserts of loose 

 drifting sand. 



The Geological Sequence. — Before attempting to show the 

 relations to one another of the different rocks which form 

 these zones, it is necessary to choose a simple standard of 

 comparison. A convenient method is to refer the African 

 rocks to their position in the sequence observed between 

 Holyhead and London. This begins at the former town with 

 some schists, and next reaches a ridge of " gneiss," which is one 

 of the oldest rocks known to the geologist. Passing thence to 

 the south-east, the traveller continually moves from older to 

 nerWer deposits. Thus after crossing the Menai Strait the 

 Archean gneisses and Cambrian slates are left behind, and to 

 the south can be seen the summits of the extinct volcanoes of 

 Snowdon. Continuing eastward, still younger rocks belonging 

 to the Silurian period ^ are seen, followed by those of the 

 Flintshire coal-field. Here the systems belonging to the " era ^ 

 of ancient life " (Palaeozoic) are left, and a few miles before 

 Chester the railway reaches the New Red Sandstone or Trias, 

 which is traversed till three miles before Rugby. There the 

 sandstones disappear below some clays, and the route crosses 

 an alternation of ridges of limestone and valleys of clay, 

 belonging to the period of the Oolitic Limestones or Jurassic. 

 These in turn are succeeded by the rocks of the Cretaceous 

 period ; of these the best known member is the Chalk, which 

 can be seen on both sides of the railway, as it passes along an 

 old river valley cut through the Chiltern Hills. At Watford 

 the Chalk, the latest English representative of the " era of 



^ The terms used for divisions of time in the present chapter have a somewhat 

 technical meaning : thus, an epoch is a subdivision of a period, and this in turn is part 

 of an era. 



