May 22, 1884] 



NA TURE 



9i 



There is now only one experiment left. This relates to the 

 effect of curvature in the streams on the stability of the motion. 

 Here again we see the whole effect altered by apparently a very 

 slight cause. If the water be flowing in a bent channel in 

 steady streams, the question as to whether the motion will be 

 stable or not turns on the variation of the velocity across the 

 channel. In front of the lantern is a cylinder with glass ends, 

 so that the light passes through in the direction of the axis. The 

 cylinder is full of water, the disk of light on the screen being 

 the light which passes through this water, and is bounded by the 

 circular walls of the cylinder. By means of two tubes tempo- 

 rarily attached, a stream of colour is introduced so as to form a 

 colour-band right across the cylinder, extending from wall to 

 wall ; the motion is very slow, and, the taps being closed and 

 the tubes removed, the colour-band is practically stationary. 

 The vessel is now caused to revolve about its axis. At first 

 only the walls of the cylinder move, but the colour-band shows 

 that the water gradually takes up the motion, the streak being 

 wound off at the ends into two spiral lines, but otherwise re- 

 maining -till and vertical ; when the streak is all wound off and 

 the spirals meet in the middle, the whole water is in motion. 

 But as the vessel is revolving, the motion is greatest at the out- 

 side, and is thus stable. There are no eddies, although the 

 spiral lings are so close as nearly to touch each other. The 

 vessel stops, and gradually stops the water, beginning at the out- 

 side. If tliis went on steadily, the spirals would be unwound 

 and the streak restored : but as the velocity is now greater 

 towards the centre, the motion is unstable for some distance 

 from the outside, and eddies form, breaking up the spirals for a 

 certain distance towards the middle, but leaving the middle 

 revolving steadily. Besides indicating the effect of curvature, 

 this experiment neatly illustrates the action of the earth's surface 

 on the air moving over it. the variation of temperature having 

 much the same effect on the stability of the moving fluid as the 

 curvature of the vessel. The moving air is unstable for a few 

 thousand feet above the earth's surface, and the motion conse- 

 quently sinuous to this height. The mixing of the lower and 

 upper strata produces the heavy cumulus clouds, but above this 

 the influence of the temperature predominates ; the motion is 

 stable, and clouds, if they form, are stratus, like the inner spirals 

 of the colour-bands. 



REPORT ON ATMOSPHERIC SAXD-DUST 

 FROM UNALASKA l 

 T^HE specimen of sand which fell during a rain-storm, 

 October 20, 1S83, at Unalaska Alaska, lias been sub- 

 mitted to microscopical analysis, and found to be undoubtedly of 

 volcanic origin. It is gray, and the grains are rather uniform 

 in size, rarely attaining a diameter of 035 mm. Under a hand 

 lens can be distinguished light-coloured crystals and fragments 

 which are occasionally glassy in lustre, mixed with others of 

 darker colours ; both are more or less dusty in appearance from 

 the presence of finer particles. For convenience of manipulation 

 and preservation, as well as to render the optical tests more 

 definite and decisive, the sand was mounted 111 Canada balsam 

 upon glass slides, after the manner of thin sections of rocks for 

 microscopical investigations. It is composed chiefly of either 

 broken or complete crystals of feldspar, augite, hornblende, and 

 magnetite, with numerous fragments of ground-mass and a few 

 small particles of glass freighted with grains of iron oxide or 

 other heavy minerals. The feldspar frequentl) occurs in well- 

 preserved crystals. Cleavage plates are common, but irregular 

 fragments predominate. A few thin cleavage lamella? parallel 

 to the base between crossed nicols show no bending due to 

 polysynthetical twinning, and extinction takes place when the 

 lines which indicate the clinopinacoidal cleavage are parallel to 

 the principal section of either nicol. While it is evident that 

 such thin plates are orthoclase, the prevailing feldspar is un- 

 doubtedly basic plagioclase, for chemical analysis shows the sand 

 to contain only 52-48 per cent. SiO s . The perfect crystals are 

 usually about 0-15 x 013 mm. in size, and slightly tabular, 

 parallel to the clinopinacoid. At times they present an almost 

 hexagonal aspect, and generally contain inclusions so abundantly 

 as to render the middle portion feebly translucent. Among the 

 imprisoned particles may be recognised hornblende microlites, 

 grains of iron oxide with crystallites of an indeterminable nature, 

 and their arrangement frequently imparts a distinct zonal struc- 

 ture to the feldspar. The hornblende, which is not nearly as 

 * By J. S. Diller, Ass slant Geologist, United States Geological Survey. 



prominent a constituent of the sand as the feldspar, occurs chiefly 

 in cleavage plates and irregular angular fragments. It has a 

 brown to dark brown colour, with deep absorption and strong 

 pleochroism, as in the andesite which it characterises. The size 

 of the hornblende fragments varies within small limits, averaging 

 010 x 005 mm., and the extinction angle is about g°. It 

 occasionally contains numerous crystallites arranged parallel to 

 the vertical (c") axis. In the number of slides examined several 

 brownish folia;, apparently of biotite, were observed under such 

 circumstances that their characterising optical properties could 

 not be satisfactorily determined. Of the FeMg silicates augite 

 is the most abundant. It is of a pale green colour, non-pleo- 

 chroitic, and its angle of extinction as seen in the cleavage plates 

 is about 46". Like hornblende, it is found generally in irregular 

 fragments. The prismatic fragments vary from o'io to CV35 mm. 

 in greatest length. The grains of magnetite, which may, in con- 

 siderable quantities, be readily picked out of the sand with a 

 magnet, are for the most part of irregular outline and small size. 

 Instead of forming independent grains of themselves, they are 

 generally found cleaving to fragments of the ground-mass, or 

 included in the other minerals. 



Besides the mineral ingredients already mentioned, the sand 

 contains numerous irregular grains swarming with clear crystal- 

 lites and microlites embedded in a grayish translucent to trans- 

 parent, often amorphous, base. These composite fragments cor- 

 respond to the ground-mass of the eruptive rocks to which the 

 lie sand is allied. They vary in size up to a diameter of 

 0.26 mm., and are generally rendered heavier than they would 

 otherwise be by small particles of magnetite or augite. The 

 crystal fragments frequently have portions of the ground-mass 

 attached to them, and present that ragged appearance which dis- 

 tinguishes volcanic sand from that which has been produced by 

 other methods. Feldspar, augite, hornblende, magnetite, with 

 fragments of the ground-mass, make up the bulk of the sand, 

 imposition is thai of a hornblende-andesite very like those 

 which occur at many points along tire western coast. One is 

 surprised to find a conspicuous deficiency in the most common 

 and generally prevailing element of volcanic ashes. It is true 

 that clear or sparingly microlitic glass particles are found in the 

 sand from Unalaska, but they are rather exceptional and un- 

 common. This paucity in glass fragments may be readily com- 

 prehended by reflecting upon the origin and distribution of 

 volcanic ashes. 



The United States Geological Survey party sent out last summer 

 in my charge under the direction of Capt. Dutton for the recon- 

 naissance of the southern portions of the Cascade Range, col- 

 1 lot of volcanic sand about a dozen miles north-east of 

 Mount Shasta. It does not form a thick deposit, but is wide- 

 spread over the basaltic slopes south of Sheep Rock, and like 

 that collected at Unalaska consists chiefly of crystal fragments, 

 of which feldspar is the most abundant. Hornblende, hyper- 

 sthene, augite, and magnetite are less prominent. In addition 

 to these and numerous fragments of microlitic ground-mass, there- 

 are many clear or sparingly crystalline glass particles of a 

 pumiceous character. The composition of the sand is that of a 

 hyperslhene-bearing hornblende-andesite like that which forms 

 the well-preserved and prominent crater springing up from the 

 north-western slope of Mount Shasta, about two miles from that 

 summit. This crater is the counterpart of Shasta cone, when 

 we consider (he whole volcanic pile, and has been christened 

 Shastina by Capt. Dutton to indicate the relation it bears to its 

 ic neighbour. In the volcanic sand which travelled about 

 a dozen miles north-east from Shastina, grains may be found 

 having a diameter of o - 5 mm., so that it is, on the whole, con- 

 siderably coarser and less uniform than that which fell at Una- 

 I Ictober 20, 1S83, but like the latter it is made up chiefly 

 ;ments of crystalline matter. 



On the other hand, volcanic dust which has been carried long 

 i- composed principally of glass particles, and there is 

 ncuous paucity of crystals and fragments of dense micro- 

 litic ground-mass. That which emanated from a crater in Ice- 

 land and fell over Norway and Sweden March 20 and 30, 1S75, 

 more than 750 miles from its source, is composed almost exclu- 

 sively of sharp-edged angular glass fragments with curved sides. 

 These splinters, chips, and shards of glass show by their more 

 or less curved outlines, as well as by their tubular or vesicular 

 structure, that they differ from pumice only in being fragmental. 

 In the formation of pumice the inflation anil distension by in- 

 closed steam and gases is carried so far as to produce a froth, 

 but if the same process be continued until explosion takes place, 



