7S 



HARDWICKE'S SCIENCE-GOSSIP. 



could point and say " that is the material from which 

 all sedimentary rocks have been derived," but analogy 

 leads us to suppose that if the earth had an igneous 

 origin, the original materials upon which the elements 

 first began to work were of the nature of granite or 

 basalt. From a variety of considerations drawn from 

 borings, mines, faults, natural gorges and proved 

 thicknesses of the strata of certain mountain chains, 

 the author arrives at the conclusion that the sedimen- 

 tary crust of the earth is at least of an average actual 

 thickness of one mile, and infers from the propor- 

 tionate amount of carbonates and sulphates of lime to 

 materials in suspension in various river waters flowing 

 from a variety of formations, that one-tenth of the 

 thickness of this crust is calcareous. Limestone rocks 

 have been, geology tells us, in process of formation 

 from the earliest known ages, but the extensive series 

 of analyses of water made by Dr. Frankland for the 

 Rivers Pollution Commission, shows that the later 

 strata in Great Britain are much more calcareous than 

 the earlier. The same holds true of the continent of 

 Europe, and the balance of evidence seems in favour 

 of the supposition that there has been on the whole a 

 gradual progressive increase or evolution of lime. 

 The " Challenger " soundings show that carbonate of 

 lime in the form of tests of organisms is a general 

 deposit characterising the greater part of the ocean 

 bottoms, while the materials in suspension are, 

 excepting in the case of transport by ice, deposited 

 within a distance of 200 miles of land. This wider 

 distribution in space of lime, the author thinks, must 

 also profoundly influence its distribution in time, and 

 he shows this by example and illustration. It can 

 also be proved to demonstration that the greater part 

 of the ocean bottom must at one time or another have 

 been land, else the rocks of the continents would 

 have become gradually less, instead of more, cal- 

 careous. Thus the arguments drawn from the geo- 

 graphical distribution of animals are reinforced by 

 physical considerations. The author goes on to show 

 that the area of granite and volcanic rocks in Europe 

 and the part of Asia between the Caspian and the 

 Black Sea, as shown in Murchison's map of Europe, 

 is two-twenty-fifths (^) of the whole ; much of this 

 is probably remelted sediments and some of the 

 granites the product of metamorphism. From con- 

 siderations stated at length it is estimated that the 

 area of exposures of igneous to sedimentary rocks 

 would be for all geological time liberally averaged at 

 one-tenth (■}$) of the whole. These igneous rocks are 

 either the original materials of the globe protruded 

 upwards, or they are melted sediments] or a mixture 

 of the two. The only igneous rocks we know of are 

 of the nature of granites and traps. If these rocks do 

 not constitute the substratum of the earth, and all 

 known rocks, igneous as well as sedimentary, are 

 derivative, either geological time is infinite, or the 

 rock from which they are derived is, so far as we know, 

 annihilated geologically speaking, and we have no re- 



cords of it left. If we assume the latter as true, the past 

 is immeasurable, but in order to arrive at a minimum 

 age of the earth, the author starts from the hypothesis 

 that the fundamental rocks were granitic and trappean. 

 From eighteen analyses by Dr. Frankland, it is shown 

 that the water flowing from granitic and igneous rock 

 districts in Great Britain contains on an average 373 

 parts per 100,000 of sulphates and carbonates of lime. 

 The amount of water that runs off the ground is 

 given for several of the great continental river basins 

 in Europe, Asia, Africa, and America. The annual 

 depth of rain running off the granitic and igneous 

 rock areas, taking into consideration the greater 

 height at which they usually lie and the possibility of 

 greater rainfall in earlier ages, is averaged at twenty- 

 eight inches, and the annual contribution of lime in 

 solution in the forms of carbonates and sulphates at 

 seventy tons per square mile. With these elements, 

 and giving due weight to certain physical considera- 

 tions that have been urged in limitation of the earth's 

 age, the author proceeds to his calculations, arriving 

 at this result, that the elimination of the calcareous 

 matter contained in the sedimentary crust of the 

 earth must have occupied at least 600 millions of 

 years. The actual time occupied in the formation 

 of the groups of strata as divided into relative ages 

 by Professor Ramsay, is inferred as follows : — 



Millions of years. 

 Laurentian, Cambrian, and Silurian . . . 200 

 Old Red, Carboniferous, Permian, and New- 

 Red 200 



Jurassic, Wealden, Cretaceous, Eocene, Mi- 

 ocene, Pliocene, and Post-pliocene . . 200 



600 



The concluding part of the paper consists of 

 answers to objections. The author contends that 

 the facts adduced prove geological time to be 

 enormously in excess of the limits urged by some 

 physicists, and ample to allow on the hypothesis of 

 evolution for all the changes which have taken place 

 in the organic world. 



ON ALTERNATION OF GENERATIONS. 



THIS term — used in botanical works to express a 

 consecutive series of phases exhibited in a 

 marked manner by most flowerless plants before 

 reaching maturity- — is a very unfortunate one, imply- 

 ing that each form in the series is "an individual, which 

 is erroneous. The following description of the suc- 

 cessive stages in the growth of a fern shows what the 

 term "alternation of generations" is intended to 

 convey. The spore, under favourable conditions, 

 gives origin to a minute green leaf-like body called a 

 prothallium or proembryo, bearing antheridia and 

 archegonia (the former corresponding to the stamens, 

 the latter to the pistil in flowering plants), the last 

 containing a special cell, the oospore, which after 



