134 



LECTUEES 



Fig. 12. In tlie cases in which these 



trunks and roots, in situ, are 

 found, (and they are by no 

 means uncommon,) the evidence 

 is conclusive that the coal was 

 formed on the spot where the 

 trees grew; in other words, that 

 the growth of the trees and the 

 deposit of the coal took place 

 simultaneously on the same spot. 

 This is clearly impossible in an 

 estuary, but is known to take 

 place in every peat swamp. 

 To recapitulate the whole argument : If we examine a peat bog 

 which has been for many years thickly overgrown with ferns, mosses, 

 and water plants of various kinds, and shaded by many large trees, 

 we find the soil composed entirely of black carbonaceous matter, 

 wholly destitute of structure but revealing its vegetable origin to the 

 microscope, containing fragments of trunks and branches of trees 

 lying in all possible positions, some prostrate, some inclined at all 

 angles, many, both living and dead, still erect, their roots firmly fixed 

 in the clay at the bottom of the bog, below the peaty matter which 

 has slowly gathered about their lower parts, and over the whole lie 

 thickly strewn the freshly fallen leaves. Now suppose such a peat 

 bog to be deeply buried beneath the surface of water and overwhelmed 

 with sediment of clay and sand, and again, after ages, elevated and 

 exposed by section to the scrutiny of the geologist, and we shall have 

 a complete reproduction of the phenomena of a coal seam. 



The great, and almost the only, objection which has been urged against 

 this theory is to be found, not in the phenomena of an individual coal 

 seam, but rather in the general phenomena of coal basins, in the re- 

 peated alternation in the same locality of coal seams with marine and 

 fresh water strata. We have already seen that there are in the same 

 coal basin sometimes as many as an hundred coal seams, one above the 

 other ; now, according to this theory, when the coal seam was forming 

 the spot must have been above the surface of the sea, but when the inter- 

 stratified limestones and shales were being deposited the same spot 

 must have been beneath the sea-level. Thus^ argues the objector, we 

 are driven to the enormous assumption that the same spot has been 

 successively upheaved above and depressed beneath the sea-level one 

 hundred times during the carboniferous period, and, what is still 

 more remarkable, that every time it rose above the sea it became a 

 peat swamp ; or if the intervening strata are fresh water instead of 

 marine, the difficulty seems only to be increased. 



Estuary theory. — It is to meet this very difficulty, to account 

 for this remarkable alternation of strata, that the rival theory has 

 been proposed. An estuary is the wide open mouth of a river empty- 

 ing into a tidal sea ; it is occupied sometimes by fresh and sometimes 

 by salt water. The deposit at the bottom of an estuary, in suitable 

 positions, is, therefore, an alternation of fresh water and marine strata. 

 In seasons of freshets the river water^ loaded with sediment and 



