September 2.;, 1897] 



NATURE 



499 



into two types ; that formed in the zone of flowage at 

 right angles to the stress, and due to the growth of new 

 particles and the orientation of old ones in this direc- 

 tion, thus giving rise to a grain in the rock, receives the 

 name of cleavage proper ; that which is due to definite 

 planes of parting, formed along surfaces of shear, and 

 originating in the zone of fracture, is called fissility. 

 Those rocks, which on coming to the surface from the 

 lower zone pass through the upper while under sufficient 

 pressure to produce fissility, have this structure produced 

 in the planes of true cleavage, while in other rocks it 

 arises independently of any other structures. In deep- 

 lying rocks fissility parallel to the bedding may be pro- 

 duced when the vertical weight is sufficient. Cracking 

 and deposit of minerals may cause banded structures 

 in imitation of bedding, and the imitation may be 

 strengthened by further movement in the banded rock 

 and by metamorphism. 



The origin of joints and faults is somewhat lightly 

 discussed. It is shown that both structures may result 

 from pressure or tension, and Daubree's explanation of 

 joints by torsion is shown to be but another statement of 

 their origin by complex folding. A very important 

 point is made in linking these structures together and 

 showing their relation to fissility, and it is insisted 

 that " there is every gradation between faulting and 

 fissility, and probably every gradation between faulting 

 and cleavage." 



Autoclastic rocks, crush-conglomerates and breccias, 

 are next discussed, and it is pointed out how essential 

 it is to discriminate between them and basement con- 

 glomerates. Metamorphism, in a wide sense, is next 

 dealt with under the heads of consolidation, welding, 

 cementation, injection, metasomatism, and mashing 

 (dynamic metamorphism). The origin of the chief 

 metamorphic rocks, both sedimentary and igneous, is 

 fully discussed. In dealing with the most ancient sedi- 

 mentary rocks, unconformities are of the greatest value, 

 in spite of the undeniable difficulty of finding them ; 

 other tests of age are dates of intrusion and of move- 

 ment, and number of movements undergone by different 

 members of a succession. It must here be noted, how- 

 ever, that as the brain of a crow is unable to count more 

 than three men with guns, so the brain of the geologist 

 is inadequate to count more than three directions of 

 movement, if so many. In igneous rocks the order of 

 injection is still to be regarded as the great clue. 



The pre-Cambrian rocks are divided into two great 

 groups — the basement complex, or Archaean group, and 

 the pre-Cambrian sediments, or Algonkian group. The 

 different views held as to the origin of the first group 

 are fully and fairly discussed, and after rejection of the 

 theories that they are altered sediments, intrusive rocks, 

 and primitive earth-crust, the view is provisionally 

 adopted that, while little is probably left of the primitive 

 earth-crust on which the Algonkian rocks were formed, 

 it having been destroyed by erosion, the Archai'an 

 rocks represent the plutonic rocks solidifying beneath it, 

 their formation in the upper part beginning in Archican 

 times and continuing steadfastly downwards to the pre- 

 sent day. All the intrusions of those later-formed rocks, 

 which have made their way to the surface through the 

 upper (Archaean) layer, must be separated from it and 

 considered as of later date. In dealing with the 

 Algonkian rocks, a useful reference is given to all the 

 fossils hitherto found in these rocks below the Olcnelltts 

 zone, which is taken as the base of the Cambrian. The 

 later part of Mr. Van Rise's paper will be found to be a 

 most useful summary of the present state of knowledge 

 of the American pre-Cambrian rocks. 



The appendix to this paper contains a mathematical 

 discussion of the depth of the zone of flowage, and the 

 function of stress and strain on the rocks in producing 

 cleavage and fissility. 



NO. 1456, VOL. 56] 



NOTES. 

 The following despatch from the Government of India t 

 Lord George Hamilton is published in the latest number of the 

 Kezv Bttlletiu : — »'We are informed by our Director of the 

 Botanical Survey of India that the ' Flora of British India," 

 which was begun by Sir Joseph Hooker some twenty-five years 

 ago, has just been brought by him to completion. The value of 

 j the work as a contribution to pure science has already been 

 I appreciated and acknowledged by others who are more com- 

 petent to speak in such a matter than ourselves. But we desire 

 I to express our hearty recognition of the service to India which 

 I Sir Joseph Hooker has rendered by his monumental undertaking. 

 i He has for the first time brought the botany of the Empire 

 into a collective form and placed it upon a firm and lasting basis, 

 thus completing the work which he began nearly half a century 

 ago in the Himalayas. We would ask your lordship to convey 

 to Sir Joseph Hooker our high appreciation of his labours, and 

 of their value and importance as systematising and adding to our 

 knowledge of the vegetable productions of India, and our 

 hearty congratulations upon having brought to a satisfactory 

 conclusion a work to which he has devoted so many years of 

 his life." In transmitting a copy of this letter to Sir Joseph 

 Hooker, Sir Arthur Godley writes : — " Lord George Hamilton 

 desires heartily to associate himself with the Government of 

 India in their acknowledgment of the valuable services you have 

 done to India by this great work, and by your labour in the 

 field of Indian botany, since you first visited that country 

 nearly fifty years ago." 



With reference to the foregoing note, we learn from the Kew 

 Bulletin that Sir J. D. Hooker's literary activity has not ceased 

 with the completion of the "Flora of British India," which has 

 occupied him for a quarter of a century. The veteran botanist 

 has offered to undertake the preparation of the two remaining 

 volumes of the " Handbook to the Flora of Ceylon," left un- 

 written by the untimely death of Dr. Trimen. The necessary 

 materials and specimens have already been received at Kewr 

 from the Royal Botanic Garden, Peradeniya. 



The St. John's correspondent of the Times states that Lieut^ 

 Peary, the Arctic explorer, has returned from Greenland, bring- 

 ing the Cape York meteorite, weighing forty- five tons, the 

 largest in the world, and also six Arctic Eskimos, who are going 

 polewards with him next summer. All the members of the 

 expedition are well. 



The Commission du Musee d'Histoire naturelle at Geneva 

 has formed itself into a committee having for its object the 

 erection of a monument to the memory of Francois Jules 

 Pictet -de la Rive. A site for the monument has been granted, 

 in front of the museum. Old students of the eminent investigator, 

 and all who are interested in the work which he accomplished, 

 are invited to send subscriptions for the memorial fund to MM. 

 I Lombard, Odier & C", Geneve. 



Dr. T. W. Engelmann, professor of physiology in the 

 University of Utrecht, has been appointed successor to the late 

 Prof. Du Bois-Reymond in the chair of Physiology at Berlin. 



Prof. Wiesner, of Vienna, has undertaken during the past 

 summer a journey to Spitsbergen to complete his observations, 

 previously made in the Tropics, as to the effect of light and 

 other external conditions on the growth of plants. 



An International Ornithological Congress will be opened at 

 Aix on November 9. 



It is reported that earthquake shocks were felt at Tashkent 

 on Saturday last, September 18. The disturbance was noticeable 



