DISTRIBUTE INSECT VARIETY. 



261 



At this period, by tracing the deposits of sandstones and muddy 

 limestones, replete with the earliest types of cosmopolitan shells, 

 geologists delineate an ocean studded with coral-fringed islands 

 occupying the future site of Europe and North America, whose 

 volcanoes and larva streams, seen in the conical mountain peaks, 

 corrie, and dyke, still form recognisable features in our Highland 

 scenery. The first vegetation of these islands faded on the 



DIAGRAM OF A COAL BASIN, OE ANCIENT LAKE BOTTOM. 

 Fresh-water layers — a, sandstone ; b, coal ; c, under clay or ancient soil ; d, shale and iron- 

 stone tails. Salt-water layer — e, limestone. 



stranded districts once its own, must be sought over barren and 

 thickly-populated spots where seams of sooty coal (See Fig. b) 

 constitute the perennial falls, matted leaves, seeds and bark of this 

 rustling land that, originally a raised sea-bed, again and again 

 slowly sank in some lake or river, which as the land subsided 

 buried its rotting columnar trunks of sigillariae, slimy with land 

 shells and millipeds, and its rustling beds of tall and reedy 

 calamites in drifting layers of sand [a) and clay [d) . And this 

 went on until this sediment accumulating, or raised (as has been 

 the opinion), reaches the surface,^ so as to allow another and 

 another thicket to spread from the land, spin a light tress of 



* Some have thought the deposit to have been successively raised and 

 depressed. (Yet why ?) Anniversary address of Leonard Horner, Esq., Journ. 

 Geological Soc, Vol. II., p. 171, 1846. Sir Charles Lyell," Principles of Geology. " 



