August 26, 1 8 80] 



NATURE 



593 



whether these differences were due to the presence of the liquid 

 water, or whether its presence and the characteristic structure 

 were aot both the effects of the great pressure of superincumbent 

 rocks. I do not see how this can be decided in a perfectly satis- 

 factory manner, but must confess that I am inclined to believe 

 that, whilst great pressure was necessarily the reason why the 

 water did not escape as vapour, the presence of liquid water 

 during final consolidation must have had a very considerable in- 

 fluence in modifying the structure of the rock, and had a great 

 share in developing what we may call the granitic type. 



It would be very instructive to follow out the gradual passage 

 from one extreme type to another far more completely than is 

 possible on the present occasion. The most interesting examples 

 cf rocks intermediate between the granitic and volcanic types 

 that I have been able to examine in adequate detail are the 

 various Cornish elvans and other quartz felsites, which furnish 

 all but a complete passage from pitch-stone to granite. Some 

 specimens prove that (juartz may crystallise out from and inclose 

 a perfectly glassy base, w-ithout a trace of liquid water, and at 

 the same time other specimens prove equally vi-ell that, as we 

 approach the granitic type, the quartz was not deposited from 

 a glassy solvent, but inclosed more or less water. In the few 

 intermediate cases there appears to be evidence of the conjoint 

 presence of uncombined water and melted stony matter. On 

 the whole, if we take into consideration only the external form 

 of the larger crystals, rocks of the gi'anitic type are very much as 

 though the crystals met with in truly volcanic rocks had been 

 strained out from the glassy or fine-grained base, and the 

 intermediate spaces filled witli quartz. The internal structure 

 of the crystals is however very different, the cavities in one class 

 containing glass, and in the other water. This most essential 

 and characteristic difference proves that rocks of the true granitic 

 type cannot have been formed simply by the more complete 

 crystallisation of the general base of the rock. If the crystals in 

 granite were analogous to those developed in volcanic rocks, 

 and the only essential difference were that the residue ci-ystallised 

 out more slowly and completely, so as to give rise to a more 

 coarsely crystallised base, the crystals first formed ought not, as 

 I think, to dififer so essentially as that in one case they should 

 inclose only glass, and in the other only water. Taking all into 

 consideration, we can therefore scarcely suppose that the crystals 

 in granitic rocks were deposited from a truly-melted dry glassy 

 solvent, like those in volcanic rocks or in slags. 



Gencynl Results. — I have, I tru^t, now said enough to show that 

 the objects here described may be conveniently separated into 

 three well-marked groups, viz. artificial slags, volcanic rocks, 

 and granitic rocks. My own specimens all show perfectly well- 

 marked and characteristic structures, though they are connected 

 in some cases by intermediate varieties. Possibly such connecting 

 links might be more pronounced in other specimens that have 

 not come under my notice. I must, however, base my conclu- 

 sions on V hat I have been able to study in an adequate manner, 

 by examining ray o^^ n preparations, and leave it for others to 

 correct any error into which I m.ay have been led from lack of 

 more numerous specimens. In any case the facts seem abund- 

 antly sufficient to prove that there must be some active cause for 

 such a common, if not general difference in the structural 

 character of these three different types. The supposition is 

 so simple and attractive that I feel very much tempted to suggest 

 that this difference is due to the presence or absence of water as a 

 gas or as a liquid. In the case of slags it is ;;'.'/ present in any 

 form. Considering how large an amount of steam is given off 

 from erupted lavas, and that, as a rule, no fluid cavities occur in the 

 constituent minerals, it appears to me very plausible to suppose 

 that those structures which are sfecially characteristic of volcanic 

 rocks are in great measure, if not entirely, due to the presence 

 of associated or dissolved vapour. The fluid-cavities prove that 

 water was sometimes, if not always, present as a liquid during 

 the consolidation of granitic rocks, and we can scarcely hesitate 

 to conclude that it must have had very considerable influence on 

 the rock during consolidation. .Still, though these three extreme 

 types appear to be thus characterised by the absence of water or 

 by its presence in a state of vapour or liquid, I think we are 

 scarcely in a position to say that this difference in the conditions 

 is more than a plausible explanation of the differences in their 

 structure. At the same time I do not know any facts that are 

 opposed to this conclusion, and we should perhaps not greatly 

 err in thus correlating the structures, even though the water was 

 not the essential and active cause of the differences. 



Confining our attention to the more important crystalline con- 



stituents which are common to the different types, we may say 

 that the chief structural characters of the crystals are as 

 follows : — 



a. Skeleton crystals. 



b. Fan-shaped groups. 



c. Glass cavities. 



d. Simple crystals. 



e. Fluid cavities. 



These different structural characters are found combined in 

 different ways in the different natural and artificial products, and 

 for simplicity I will refer to them by means of the affixed 

 letters. 



The type of the artificial products of fusion may generally be 

 expressed hy a -V b ot b + c ; that is to say, it is characterised by 

 skeleton crystals and fan shaped groups, or by fan-.'haped groups 

 and glass-cavities. In like manner, the volcanic type may be 

 expressed occasionally h^ b + c, but generally by c 4- d, and the 

 granitic by d -f c. These relations will be more apparent if 

 given in the form of a table as follows : — 



s'^'gfype I'^'^^-f^ 



Volcanic type j , , 



Granitic type d+e 



Hence it will be seen that there is a gradual passage from one 

 type to the other by the disappearance of one character and the 

 appearance of another, certain characters in the meanwhile re- 

 maining common, so that there is no sudden break, but an 

 overlapping of structural characteristics. It is, I think, satis- 

 factory to find that, when erupted rocks are examined from such 

 a new and independent point of view, the general conclusions 

 to which I have been led are so completely in accord with those 

 arrived at by other methods of study. 



Conclusion. — And now I feel that it is time to conclude. I 

 have necessarily been compelled )o give only a general account 

 of the subject, and perhaps for want of adtquate description 

 many facts may appear more complex than they really are. 

 Some are indeed of anything but simple character, and their full 

 explanation is perhaps beyond our present power. The greater 

 part are, however, much more simple and easy to observe than 

 to describe ; and, even if I have failed to make everything as 

 plain as I could wish, I hope that I have succeeded in making 

 the principal points sufficiently clear to show that the structure 

 of slags and analogous artificial products throws much light on 

 the structure and origin of the various groups of erupted rocks. 

 I feel that very much still remains to be learned, and, as I 

 think, could be learned by the further extension of this 

 method of inquiry. What strikes me most is the great 

 necessity for the more complete appreciation of experimental 

 methods of research ; but to carry out the experiments necessary 

 to clear up the essential difficulties of the subject would, I fear, 

 be a most difficult undertaking. In the meantime all that we 

 can do is to compare the structure of known artificial products 

 with that of natural rocks, and to draw the be--t conclusions we 

 can from the facts, as viewed in the light of our present 

 knowledge of chemisti7 and physics. My own impression is 

 that there is still much to be learned respecting the exact 

 conditions under which some of our commonest rocks were 

 formed. 



SECTION D 



BIOLOGY 



Opening Address by Dr. A. C. L. G. GiJNTHER, M.A., 

 Ph.D., F.R.S., President of the Section 



Sixteen years ago, at the meeting of the British Association 

 at Bath, the duty which I am endeavouring to discharge to-day 

 was intrusted to my predecessor and old friend, the late Dr. John 

 Edward Gray. In the address which he then delivered before 

 this section he spoke on " Museums, their Use and Improve- 

 ment ; " and he, who had devoted a whole lifetime to the forma- 

 tion and management of one of the greatest zoological collections 

 in the world, was well qualified to give an opinion and advice on 

 this subject. Indeed, when I read now what he then insisted on 

 as a necessary change in the system of museums, I feel compelled 

 to pay a passing tribute to his memory. 



Zoology, geology, botany were to him not distinct and inde- 

 pendent studies ; the views advanced by Lamarck, by Treviranus, 



