356 



NATURE 



\Feb. 2 2, 1877 



we needed large camp-fires in the morning and evening. Our 

 camp at Trout Lake could only be reached for six weeks in 

 summer on account of the depth of snow in the fallen timber. 



On the whole, it may be remarked of this mountain region, 

 that the astronomical condition, particularly for photographic 

 researches, is unpromising. In only one place were steadiness 

 and transparency combined, and only two nights out of fifteen at 

 the best season of the year were exceptionally fine. The trans- 

 parency was almost always much more marked than at the sea- 

 level, but the tremulousness was as great, or even greater, than 

 near New York. It is certain that during more than half the 

 year no work of a delicate character could be done. At the end 

 of August, in sheltered positions, and in good tents, we slept 

 under half a dozen thicknesses of blanket, and only partially 

 undressed. Such a degree of cold distracts the mind and numbs 

 the body. Apparently, therefore, judging from present informa- 

 tion, it would not be judicious to move a large telescope and 

 physical observatory into these mountains with the hope of doing 

 continuous work under the most favourable circumstances. 



TESTIMONIAL TO MR. DARWIN 



ly/TR. DARWIN has received as a testimonial, on the 

 ^^ occasion of his sixty-ninth birthday, an album, 

 a magnificent folio, bound in velvet and silver, con- 

 taining the photographs of 154 men of science in 

 Germany. The list contains some of the best known 

 and most highly honoured names in Europe. He has 

 likewise received on the same occasion from Holland an 

 album with the photographs of 217 distinguished pro- 

 fessors and lovers of science in that country. These 

 gifts are not only highly honourable to Mr. Darwin, but 

 also to the senders as a proof of their generous sympathy 

 with a foreigner ; and they further show how widely 

 the great principle of Evolution is now accepted by 

 naturalists. 



A German correspondent informs us that the German 

 album bears on the handsome title-page the inscrip- 

 tion "Dem Reformator der Naturgeschichte, Charles 

 Darwin." 



MICROSCOPICAL INVESTIGATION OF SANDS 



AND CLAYS ^ 

 HTHE anniversary address of the president, Mr. H. C. 

 ■^ Sorby, F.R.S., at the Royal Microscopical Society on 

 Wednesday, March 7, consisted mainly of an attempt to 

 treat in a systematic manner the application of the micro- 

 scope to the study of the mineral constituents of sands 

 and clays. The various organisms found in such deposits 

 have been much studied by Ehrenberg and other micro- 

 scopists, and of late years much attention has been 

 directed to the structure of igneous and other hard rocks, 

 more or less allied to them, which can be cut into thin 

 sections ;_ but comparatively little attempt has been made 

 to investigate the ultimate constitution of loose sands, 

 muds, and clays. 



The scope of this subject, as treated by the author, in- 

 cluded the identification of the true mineral nature of the 

 various particles, and the determination of the nature of 

 the rock from which they were originally derived ; the 

 chief aim being to trace back the history of the material 

 to the furthest possible extent. 



After describing the manner in which the different 

 kinds of deposits should be prepared, examined, and 

 mounted as permanent objects, the author treated at 

 some length on the conditions necessary for satisfactorily 

 seeing the various particles with moderate or very high 

 magnifying powers, and for observing their microscopic 

 structure and optical characters. The particles of clay 

 and the fluid-cavities in the grains of sand are often so 

 minute as to task the power of the microscope to the 

 fullest extent, and some indeed are so small that their 

 perfect definition may perhaps be impossible by any 

 means at our command. It was shown that the condi- 



» Abstract by the author. 



tions under which many of the objects are visible are 

 such that with highly convergent light and object-glasses 

 of large aperture no dark outline is possible, and therefore 

 they are quite invisible, but become quite distinct when 

 the aperture is reduced to a moderate and appropriate 

 amount. For this reason object-glasses of comparatively 

 small aperture are far the best, since the focal point being 

 further from the front lens, very high powers can be used 

 in cases which are beyond the reach of lenses of large 

 aperture. 



The author then went into much detail to show the 

 character of the grains of quartz, mica, and other minerals 

 derived from the decomposition or breaking up of various 

 crystalline rocks, and showed that on the whole there are 

 many characteristic differences between the material de- 

 rived from granitic and schistose rocks — this difference 

 consisting mainly in the form, internal structure, and 

 optical characters of the various constituent grains ; the 

 general conclusion being that a careful study of sands, 

 muds, and clays enables us to form a very satisfactory 

 opinion as to whether they were derived mainly from 

 granitic or schistose rocks, or from a mixture of the two 

 in some approximately definite proportion. It was also 

 shown that the shape of the particles as originally derived 

 from their parent rock is sufficiently definite and charac- 

 teristic to enable us to form a very good opinion respect- 

 ing the amount of subsequent mechanical or other change. 



Applying those principles to the study of particular 

 typical cases, it was shown that the coarser grained 

 British sandstones have been mainly derived from granite 

 rocks, of a character somewhat intermediate between 

 those of the Scotch Highlands and Scandinavia. Some 

 of these sandstones consist of grains which have under- 

 gone scarcely any weaiing, and are as angular as those 

 derived directly from decomposed granite, and are thus 

 totally unlike the blown sand of the deserts, which are 

 worn into perfectly rounded grains. 



The finer grained sands are no less angular than the 

 coarse, and have not been derived from the wearing 

 down of larger fragments, but have resulted from the 

 separation of the small from the large grains by the 

 action of currents. Though some fine-grained sand-stones 

 have been mainly derived from granitic rocks, yet, on the 

 whole, the small particles of quartz have more commonly 

 been derived from the breaking up of schistose rocks. 

 Clays and shales consist to a great ^extent of particles 

 identical in all their characters with those derived from 

 the decomposition of felspars and other minerals which 

 undergo a similar change. As a general rule we meet 

 with many grains of sand even in clays chiefly consisting 

 of extremely minute granules, which can easily be ex- 

 plained by the remarkable manner in which such mate- 

 rial, when suspended in water, collects into small compound 

 grains, which subside at a rate quite independent of what 

 would be the velocity of subsidence of the separate 

 particles if they were detached. 



The conclusions derived from a study of the characters 

 of the separate grains are confirmed by the occurrence of 

 what may be truly considered to be grains of granite or 

 mica schist. We also in some cases meet with grains 

 sufficiently large to show the characteristic structure of 

 the still more complex rocks of which they are composed. 

 Thin sections of some of the oldest slates in Wales are 

 thus as it were a perfect museum of specimens of the 

 rocks existing at a still earlier period, broken up and 

 worn down into the sands which formed these very 

 ancient slates. 



In order to establish these various conclusions it would 

 be necessary to enter into a large amount of detail, but 

 perhaps what has been said may suffice to indicate the 

 general line of inquiry, and to show that by making full 

 use of every microscopic means, it is possible to learn 

 many important facts from such very unpromising mate- 

 rials as sands and clays, 



• 



