ORGANIC AGENCIES. 43 



not to pressure alone, nor to volcanic heat alone, that the 

 solid strata, originally of sand, gravel, mud, and organic 

 debris, owe their hardness and crystalline texture. Chemi- 

 cal infiltrations and combinations are everywhere as opera- 

 tive as these are, and, indeed, in most instances are the main 

 modifiers of mineral texture, colour, and consistency. And 

 the veins and veinstones the great repositories of the 

 metallic ores that traverse the older formations, they, too, 

 are the immediate products of chemical segregation, slowly, 

 and silently, but ever at work in these secret recesses. On 

 the whole, chemical actions and reactions within the rocky 

 crust of the earth are incessant, either dissolving and dis- 

 placing, reconstructing into other forms, or aggregating in 

 other and newer compounds. 



The Organic agents fall next to be considered, and of these, 

 as of many other departments of nature, it may be remarked 

 that the minute and unobserved are the most active and 

 effective. It is true that the trees of the forest may be im- 

 bedded in peat-bogs or drifted into the mud of estuaries, 

 and that the bones of fishes, reptiles, birds, and mammals 

 may be entombed in the sediments of lakes and seas ; but, 

 on the whole, these constitute a small proportion of the con- 

 taining strata, and even where swept by currents into special 

 shoals and bone-beds, as we know they are in certain parts 

 of the ocean, they form but insignificant accumulations 

 compared with those resulting from the myriad-growths of 

 microscopic organisms. In the vegetable world the plants 

 of the peat-moss and swamp-growth claim our first attention, 

 as out of the constituents of the air and water they elaborate 

 their own substance, and year after year bequeath it to the 

 accumulating mass. How thin soever may be the film that 

 the growth and decay of a single year may add to the ac- 

 cumulation, yet in the course of centuries the peat-growth 

 and swamp-growth thicken, till now within all the colder 



