July 27, 1888.I 



SCIENTIFIC NEWS. 



89 



writers. The larvas had six legs, which were increased 

 to eight on their attaining the adult state. After passing 

 the larva stage the nymph was supposed to be parasitic 

 on some of the water-beetles ; but so little was known as 

 to this that both the Hydrachna, or scarlet mite, and 

 the Limnesia, or bog mite, afforded scope for practical and 

 interesting work. The larva of the Gyrinns, a beautiful 

 object, with branchia similar to the may-fly, would also 

 repay observation. It kept close to the bottom of ponds, 

 and was therefore not readily met with. 



The president recorded his observations of an amaeba, 

 which divided into two parts. These eventually again 

 joined and formed one animal. 



Mr. Groves stated he had at first experienced difficulty 

 in keeping chara for continuous observation, a tempera- 

 ture of over 70 degs. being fatal to the plant. By placing it 

 in a very cool place, however, he had found it live and 

 thrive. Having felt the difficulty of dissecting under the 

 ordinary microscope, in which ^everything was reversed, 

 he had adopted Mr. Stevenson's binocular, a form of 

 which he exhibited, with large rests at the side for 

 steadying the hands. 



In reply to Mr. Sebastian Davis, who thought it 

 desirable attention should be directed to the causes 

 governing the group of special forms in diffeient ponds, 

 Mr. Groves stated it was interesting to watch the gradual 

 development of vegetation in the pools of water that 

 formed in newly-opened gravel pits. During the first 

 year little else appeared than confervoid algse ; in the 

 second, chara or nitella would be found ; in the third, 

 the water buttercup perhaps made its appearance ; then 

 some of the filamentous-leaved water plants, the chara or 

 nitella being completely ousted from the pool ; whilst the 

 last to appear was the pond weed, with its flat leaves 

 resting on the surface of the water. He presumed that 

 with this succession of plants there was also a corre- 

 sponding development of animal life. 



THE ARTIFICIAL REPRODUCTION OF 

 VOLCANIC ROCKS. 



Translation of a Lecture Delivered by M. Alphonse 

 Renard, LL.D., at the Royal Institution. 



AT first sight the study of the earth's crust appears, if 

 one may so express it, entirely utilitarian. It 

 almost seems as though it were absolutely forced on 

 man by the necessity of exploring the strata of the earth 

 for the purpose of extracting minerals, building materials, 

 combustible matters, etc. 



It is evident to anyone who casts a glance at the 

 history of any of the sciences, that these sciences owe 

 their origin in the first place to a definite, practical, and 

 useful aim, that they have all passed through this initial 

 phase, and have afterwards followed a regular develop- 

 ment, the progress of which, as concerns geology, we will 

 here endeavour to trace. 



Man commences the exploration of terrestial depths in 

 order to draw from them the materials necessary to 

 supply his several wants. At first he does this at 

 haphazard ; but, as the miner's art develops, the search 

 for mineralogical treasures is carried out with more 

 method — he begins to observe the conditions under which 

 minerals and useful rocks are found in close proximity 

 in the depths of the earth. He does not hesitate to 

 generalise from his observations and from the localities 

 in which these rocks are found, and thus discovers the 



salient points of the architecture of our globe. In 

 excavating into the bowels of the earth, one is forcibly 

 convinced that the globe was not made at one fell swoop 

 but owes its formation to successive epochs. 



It is clear, therefore, that to rightly interpret the 

 history of the earth, and the part played by various 

 causes in its formation, it is necessary to make exhaus- 

 tive studies of the solid crust, so as to be able to picture 

 the state of our planet in past ages. By comparing the 

 different strata of the globe with the formations which 

 are developing before our eyes, we can succeed in re- 

 tracing the conditions which were the chief agents in the 

 formation of the layers of ancient periods. Thus, by 

 the analysis of facts and the induction from generalised 

 observations, the knowledge of the earth's crust enters 

 into a new and truly scientific phase. The original 

 attempt to discover some practical rule for the miner 

 resolves itself gradually into a desire to unravel the 

 history of the earth. A fundamental principle guides 

 us in this reconstruction of the past of our planet — the 

 fact that the essence of the forces which have acted upon 

 the earth still remains unchanged. We must seek in 

 geological epochs for the traces of those phenomena only 

 whose nature is akin to that of phenomena which we 

 ourselves witness, and which we can submit to critical 

 direct observation, A vast number of documents on the 

 history of our planet has accumulated since the latter 

 end of the last century, when the inductive method was 

 first applied to the study of the mineralogical masses 

 forming the crust of the globe, to their architecture, and 

 to the beings whose remains are fossilised in the differ- 

 ent layers. 



Let us see how, relying on this induction and putting 

 this analytical method into practice, geology interprets 

 the formation of rocks. Rocks are, as we know, the 

 solid mineral masses which constitute the strata. 

 Observation teaches us to distinguish a first group, 

 characterised by the disposal in layers or in beds ; 

 these are sedimentary rocks. A second group, which 

 does not show this stratified disposal, includes the rocks 

 of a volcanic nature and massive structure. The widely- 

 differing structures and compositions of these two great 

 lithological divisions, lead us to the conclusion that they 

 were formed under special conditions which have left their 

 distinctive marks on each group. 



The formation of the sedimentary rocks is at once 

 understood if we observe how both river and marine 

 currents deposit on their beds pebbles, sand, clay, etc. 

 As the organisms living in these waters die away, their 

 skeletons or their shells mingle with the mineral sub- 

 stances, and, with these latter, gradually raise the layer 

 of sediments. The matter thus deposited naturally takes, 

 from the manner in which it accumulates, a stratified 

 appearance. All the particles of which these layers con- 

 sist were originally isolated grains, and still bear traces 

 of their origin ; they are the remains of organisms, or 

 the fragments of pre-existent rocks, which will subse- 

 quently all be unified again by physical and chemical 

 action. 



Now let us compare these modern sedimentary de- 

 posits, characterised by their stratified disposal, and the 

 detrital nature of their constituents, with certain layers 

 of geological formation. We see, on the surfaces of 

 continents, masses owing their formation to geological 

 periods, which present close analogies of aspect and 

 structure with the materials which are deposited before 

 our eyes by river and sea-water. This comparison leads 



