106 CRITIQUES AND ADDRESSES. [v. 



lance and Calamites were not, as often supposed, composed wholly, or 

 even principally, of lax and soft tissues, or necessarily short-lived. 

 The former had, it is true, a very thick inner bark ; but their dense 

 woody axis, their thick and nearly imperishable outer bark, and their 

 scanty and rigid foliage, would indicate no very rapid growth or decay. 

 In the case of the Sigillarice, the variations in the leaf-scars in 

 different parts of the trunk, the intercalation of new ridges at the 

 surface representing that of new woody wedges in the axis, the trans- 

 verse marks left by the stages of upward growth, all indicate that 

 several years must have been required for the growth of stems of 

 moderate size. The enormous roots of these trees, and the condition 

 of the coal-swamps, must have exempted them from the danger of 

 being overthrown by violence. They probably fell in successive 

 generations from natural decay ; and making every allowance for other 

 materials, we may safely assert that every foot of thickness of pure 

 bituminous coal implies the quiet growth and fall of at least fifty 

 generations of Sigillarice, and therefore an undisturbed condition of 

 forest growth enduring through many centuries. Further, there is 

 evidence that an immense amount of loose parenchymatous tissue, and 

 even of wood, perished by decay, and we do not know to what extent 

 even the roost durable tissues may have disappeared in this way ; so 

 that, in many coal-seams, we may have only a very small part of the 

 vegetable matter produced." 



Undoubtedly the force of these reflections is not 

 diminished when the bituminous coal, as in Britain, 

 consists of accumulated spores and spore-cases, rather 

 than of stems. But, suppose we adopt Principal Dawson's 

 assumption, that one foot of coal represents fifty genera- 

 tions of coal plants ; and, further, make the moderate 

 supposition that each generation of coal plants took ten 

 years to come to maturity then, each foot-thickness of 

 coal represents five hundred years. The superimposed 

 beds of coal in one coal-field may amount to a thickness 

 of fifty or sixty feet, and therefore the coal alone, in that 

 field, represents 500 x 50 - 25,000 years. But the 

 actual coal is but an insignificant portion of the total 

 deposit, which, as has been seen, may amount to between 

 two and three miles of vertical thickness. Suppose it 

 be 12,000 feet which is 240 times the thickness of the 



