94 THE PRIMARY PERIODS. 



find interstratified overflows of lava and showers of vol- 

 canic ashes, indicating that then, as now, the vulcanic forces 

 were active in their work of upheaval and eruption. How 

 wonderfully well these old rocks have retained the record 

 of their history ! Here shore-formed sandstones pattered 

 by crustacean feet and riddled with worm-burrows ; there 

 limestones formed of coral-growth and shell-debris in the 

 deeper waters : here slaty shales composed of the slimy 

 mud of the stiller depths, and replete with zoophytes ; and 

 there at intervals vast sheets of tuff and porphyry that 

 had been showered abroad as ashes or ejected as lava from 

 submarine volcanoes. Occasionally, in the pebbles of the 

 conglomerates, we catch glimpses of the kind of rocks that 

 formed the lands from which these sediments were trans- 

 ported, and from drifted clubmoss-like twigs in the shales 

 we know that these lands were clothed in some degree with 

 a vegetation, however lowly. 



Such is the tale told of the Silurian epoch by its own rock- 

 formations ; but the history receives a deeper and livelier 

 interest when we come to consider the number and variety 

 of organisms imbedded throughout the system. It is true 

 that these are of lowly orders and wholly invertebrate, if 

 we except a few scattered fish-remains found in the very up- 

 permost beds, and by many regarded as belonging more pro- 

 perly to the Old Eed Sandstone ; but lowly as they are, they 

 occur in vast exuberance and variety, and mark a marvellous 

 progress in life-development compared with what is known 

 of the Cambrian and Laurentian. Sea-weeds and drifted 

 twigs and spores of clubmoss-like land-plants is all we 

 know of the Silurian flora ; but certain beds of anthracite 

 and anthracitous shales favour the idea that plant-life was 

 then more exuberant than has yet been detected. In its 

 fauna or animal -life we have foraminiferal organisms, 

 sponges, corals, polyzoa, or aggregate animals like the sea- 



