154 LECTDEE8 



be rejected from the equation." True, but infinitely small quantities 

 accumulating through infinite ages become finite, in fact, become very 

 important ; for it is these very same infinitely small residual quanti- 

 ties, rejected by astronomy as of no value, which, by their accumula- 

 tion, constitute the progressive development of the earth and solar 

 system. Without such small uncompensated forces history, whether 

 geological, national, or individual, would be impossible. An insect 

 philosopher, the span of whose life is a single day, attentively ob- 

 serving the daily cycle of the healthy human body, might rationally 

 assert the stability of the human system. The body, at the end of 

 twenty-four hours, has come back to the same spot whence it started. 

 At least the variation, if any, must be infinitely small, and therefore, 

 for all purposes of insect life, may be rejected as of no value. And 

 yet it is the accumulation of this same infinitely small variation which 

 constitutes the growth and progressive development of the body. This 

 is not an exaggerated illustration, for 2,000 years, the whole age of 

 astronomy, is but one day, yea, but a small fraction of a day, in the 

 geological history of the earth. 



The flora of the coal period is more complete than that of any pre- 

 vious or succeeding geological epoch. The whole number of fossil 

 species of plants known is probably not far from 2,000. Of these, 

 according to estimates made more than ten years ago, about 816 are 

 from the " coal measures." The constant additions which have been 

 made since that time, particularly by Dr. Newberry and others, from 

 an examination of the coal fields of our own country, would probably 

 bring the number up to at least 900. Probably, therefore, nearly if 

 not quite one-half of all known fossil plants belong to this period. I 

 have already said that a coal seam is made up of the remains of such 

 plants, yet it is not in the coal seams themselves that we find the best 

 preserved specimens of coal plants. On the contrary, the vegetable 

 matter is here so thoroughly disorganized that it is only by means of 

 the microscope that we are able to detect its original structure. It is 

 rather in the associated shale strata that the most beautiful impres- 

 sions occur, particularly in the overlying hlach slate. Between the 

 thin sheets of this slate the stems and leaves are as perfiectly preserved, 

 every vein and nerve, as between the leaves of the botanist's herbarium. 

 This fact, viz : that the well-preserved plants are always found in 

 abundance in this position, and never in the coal seam proper has, as 

 it seems to me, an important bearing upon the theory of coal de- 

 posit. But of this we shall speak again in another place. You have 

 here before you a magnificent slab of black slate, profusely covered 

 with beautiful impressions of leaves and stems of ferns and calamites. 

 In this case, as perhaps in most others, the impressions, though well- 

 defined, are not conspicuous at a distance, because the color of the 

 ground and of the figures are so nearly alike, but in some cases, when 

 the shale background is light-colored, the relief of the coal-black 

 impressions is very beautiful. The newly exposed roof of a coal mine 

 has been compared by Dr. Buckland to the most magnificent fresco 

 painted ceilings of Italian buildings. 



But although the number of species of coal plants is so great, yet 



