34 



NATURE 



{_Nov. 9, 1871 



there been shown by Daubree that the elements of the zeolites 

 had been derived in part from the waters, and in part from 

 the mortar, and even the clay of the bricks, which had 

 been attacked, and had entered into combination with 

 the soluble matters of the water to form chabazite. I, 

 however, at the same time pointed out another source of 

 silicated minerals, upon which I had insisted since 1S57, viz , 

 the reaction between silicious or argillaceous mitters and earthy 

 carbonates in the presence of alkaline solutions. Numerous 

 experiments showed tliat when solutions of an alkaline carbo- 

 nate were heated with a mixture of silica and carbonate of mag- 

 nesia, the alkaline silicate formed acted upon the latter, yielding 

 a silicate of magnesia, and regenerating the alkaline carbonate ; 

 which, wiihout entering into permanent combination, was the 

 medium through which the union of the silica and the magnesia 

 was effected. In this way I endeavoured to explahi the altera- 

 tion, in the vicinity of a great intrusive mass of dolerite, of a 

 gray Silurian limestone, which contained, besides a little car- 

 bonate of magnesia and iron-oxyd, a portion of very silicious 

 matter, consisting apparently of comminuted orthoclase and 

 quartz. In place of this, there had been developed in the lime- 

 stone, near its contact witli the dolerlle, an amorphous greeniih 

 basic silicate, which had seemingly resulted from the union of 

 the silica and alumina with tlie iron-oxyd, the magnesia, and a 

 portion of lime. By the crystallisation of the products thus 

 generated it was conceived that minerals like hornblende, garnet, 

 and epidote might be developed in earthy sediments, and many 

 cases of local alteration explained. Inasmuch as the reaction 

 described required the intervention of alkaline solutions, rocks 

 from which these were excluded would escape change, although 

 the other conditions might not be wanting. The natural associa- 

 tions of minerals, moreover, led me to suggest that alkaline 

 solutions might favour the crystallisation of aluminous silicates, 

 and thus convert meclianical sediments into gneisses and mica- 

 schists. The ingenious experiments of Daubree on the part 

 which solutions of alkaline silicates, at elevated temperatures, 

 may play in the formation of crystallised minerals, such as feld- 

 spar and pyroxene, were posterior to my early publications on 

 the subject, and fully justified the importance which, early in 

 18571 I attributed to the intervention of alkaline silicates in the 

 foimation of crystalline silicated minerals.* 



While, however, there is good reason to believe that solutions 

 of alkalrae silicates or carbonates have been efficient agents in the 

 crystiliisation and molecular re-arrangement of ancient sediments, 

 and have also played an important part in the local alteration of 

 sedimentary strata which is often observed in the vicinity of in- 

 trusive rocks, it is clear to me that the agency of these solutions is 

 less universal than was once supposed by Daubree and myself, and 

 will not account for the formation of various silicated rocks found 

 among crystaUine schists, such as serpentine, hornblende, steatite, 

 and chlorite. When I commenced the study of these crystalline 

 strata, I was led, in accordance with the almost universally re- 

 ceived opinion of geologists, to regard them as resulting from a 

 subsequent alteration of pal.-eozoic sediments, which, according to 

 different authorities, were of Cambrian, Silurian, or Devonian 

 age. Thus in the Ap]3alachian region, as we have already seen, 

 they have, on supposed stratigraphical evidence, been successively 

 placed at the base, at the summit, and in the middle of the 

 Lower Silurian or Champliin division of the New York system. 

 A careful chemical examination among the unaltered paloeozoic 

 sediments, which in Canada were looked upon as the strati- 

 graphical equivalents of the bands of magnesian silicates in these 

 crystalline schists, showed me, however, no magnesian rocks 

 except certain silicious and ferruginous dolomites. From a con- 

 sideration o! reactions which I had observed to take place in such 

 admixtures in presence of heated alkaline solutions, and from 

 the composition of the basic silicates which I had found to be 

 formed in silicious limestones near their contact with eruptive 

 rocks, I was led to suppose that similar actions, on a grand scale, 

 might transform these silicious dolomites of the unaltered strata 

 into crystalline magnesian silicates. 



Further researches, however, convinced me that this view was 

 inapplicable to the ciystalline schists of the Appalachians ; since, 

 apart from the geognostical considerations set forth in the ]ire- 

 vious part of this paper, I found that these same ci-ystalline strata 

 hold beds of quailzose dolomite and magnesian carbonate, asso- 

 ciated in such intimate relations with beds of serpentine, diallage, 

 and steatite, as to forbid the notion that these silicates could have 



• Proc Roy. Soc, May 7, 1857. Amer. Jour. Set., u. xxiii. 438, ami 

 Kxv. 289 and 435. 



been generated by any transformations or chemical re-arrange- 

 ment of mixtures like the accompanying beds of quartzose 

 magnesian carbonates. Hence it was that already, in 1S60, as 

 shown above, I announced my conclusion that serpentine, chlo- 

 rite, and steatite had been derived from silicates like sepiolite, 

 directly formed in waters at the earth's surface, and that the 

 crystalline schists had resulted from the c ->nsolidation of previ- 

 ously formed sediments, partly chemic.il anJ partly mechanical 

 in their origin. The latter being chiefly silico-aluminous. took, 

 in part, the forms of gneiss and mica-schists, while from the more 

 argillaceous strata, poorer in alkali, much of the ahiminous 

 silicate ciystallised as andalusite, staurolite, cyanite, and garnet. 

 These views were reiterated in 1863,* and further in 1S64, in the 

 following language, as regards the chemically-formed sediments : 

 " steatite, serpentine, pyroxene, hornblende, and in many cases, 

 garnet, epidote, and other silicated minerals are formed by a 

 crystallisation and molecular re-arrangement of silicates generated 

 by chemical piocesses in waters at the earth's surface. "+ Their 

 alteration and crystallisation were compared to that of the me- 

 chanically formed feldspathic, silicious, and argillaceous sediments 

 jusi mentioned. 



( To be continuid. ) 



THE RELATIONS BETWEEN ZOOLOGY 

 AND PAL.EONTOLOGYt 

 1\ /T Y distinguished predecessor, the late Prof. E. Forbes, appears 

 ^^ ^ to have been the first who undertook the systematic study of 

 nnrine zoology with reference to the distribution of marine animals 

 in space and in time. After making himself well acquainted with 

 the fauna of the British seas to the depth of about 200 fathoms 

 by dredging, and by enlisting the active co-operation of many 

 friends, among whom we find Mac.\ndrew, Barlee, Gwyn 

 Jeffrej s, William Thompson, and many others, entering enthusias- 

 tically into the new field of natural history inquiry ; in the year 

 1S41, Forbes joined Captain Graves, who was at that time in 

 command of the Mediterranean Survey as naturalist. During 

 about eighteen months he studied with the utmost care the con- 

 ditions of the j'Egean and its shores, and conducted upwards of 

 100 dredging operations at depths varying from I to 130 fathoms. 

 In 1843 he communicated to the Cork meeting of the British 

 Association an elaborate report on tlie mollusca and radiata of 

 the /Egean Sea, and on their distribution as bearing on geology. 

 Three years later, in 1846, he published in the first volume of 

 the " Memoirs of the Geological Survey of Great Britain," a 

 most valuable memoir upon the connection between the existing 

 Fauna and Flora of the British Isles and the geological changes 

 which have affected their area, especially during the epoch of 

 the northern drift. In the year 1S59 appeared the "Natural 

 History of the European Seas," by the late Prof. Edward 

 Forbes, edited and continued by Robert Godwin-Austen. 

 In the first hundred pages of this little book Forbes gives 

 a general outline of some of the more important of 

 his views with regard to the distribution of marine forms. 

 The remainder of the book is a continuation by his friend Mr. 

 Godwin- Austen, for before it was finished an early death had cut 

 short the career of the most accomplished and original naturalist 

 of his time. I will give a brief sketch of the general result to 

 which Forbes was led by his labours, and I shall have to point 

 out that, although we are now inclined to look somewhat diffe- 

 rently on certain very fundamental points, and, although recent 

 investigations with better appliances and more extended ex- 

 perience have invalidated many of his conclusions, to Forbes is 

 due the credit of having been the first to treat these questions in 

 a broad pliilosonhical Sense, and to point out that the only means 

 of acquiring a irue knowledge of the rationaU of the distiibu- 

 tion of our present fauna is to make oursehes acquainted with 

 its history, to connect the present with the past. This is the 

 direction which must be taken by future inquiry :— Forbes as 

 a pioneer in this line of research was scarcely in a position to 

 appreciate the full value of his work. Every year adds enor- 

 mously to our stock of data, and every new fact indicates more 

 and more clearly the brilliant results which are to be obtained by 

 following his methods, and by emulating his enthusiasm and his 

 indefatigable industry. Forbes believed implicitly, alonij with 

 nearly all the leading naturalists of his time, in the immutability 



* Geol. of CaiLida, pp. 577 — 581. 

 t Amcr Jour. Sci., H. .v.xxvii. 266, and xx,xviii. 183. 



X Abstract of Opening Lecture on Natural History delivered at the Uni- 

 versity of Edinburgh, Nov. z, by Prof. Wyville Thomson, F.R.S. 



