August -6, iSSo] 



NATURE 



391 



through modern and more ancient volcanic to plutonic rocks, in 

 such a manner as to show at once that they are intimately related 

 and yet differ in such characteristic particulars that I think other 

 agencies than mere heat must have had great influence in 

 producing the final results. 



In dealing with this subject 1 propose in the first place to de- 

 scribe the characteristic structure of products formed artificially 

 under perfectly well-known conditions, and then to pass gradually 

 to that of rocks whose origin must be inferred, and cannot be 

 said to have been completely proved. 



Crystalline Blj-afipe Beads. — Some years ago I devoted a con- 

 siderable amount of time to the preparation and study of 

 crystalline blowpipe beads, my aim being to discover simple and 

 satisfactory means for identifying small quantities of different 

 earths and metallic oxides, when mixed with others, and I never 

 supposed that such small objects would throw any light on the 

 structure and origin of vast ma-ses of natural rock. The manner 

 in which I prepared them was as follows . — A small bead of 

 borax was so saturated with the substance under examination at 

 a high temperature that it became opaque either on cooling or 

 when slowly re-heated. It was again fused so as to be quite 

 transparent, and then very slowly cooled over the flame. If 

 properly managed, the excess of material held in solution at a 

 high temperature slowly crystallised out, the form and character 

 of the crystals depending on the nature of the substance and en 

 the presence of other substances added to the bead as test re- 

 agents. By this means I proved that in a few exceptional cases 

 small simple solid crystals are formed. More frequently they 

 are compound, or occur as minute needles, but the most charac- 

 teristic peculiarity is the development of complex skeleton 

 crystals of extreme beauty, built up of minute attached prisms, 

 so as to give rise to what would be a well-developed crystal with 

 definite external planes, if the interspaces were all filled up. 



In many cases the fibres of these skel jtons are parallel to three 

 different axes perpendicular to one another, and it might be sup- 

 posed that the entire skeleton was due to the growth of small 

 needle-shaped crystals, all uniformly elongated in the line of one 

 crystalline axis, so that the resulting mass would be optically 

 and crystogi-aphically complex ; but in some cases tlie different 

 systems of fibres or needles are inclined obliquely, and then the 

 optical characters enable us to prove that the separate prisms are 

 not similar to one another, but developed along different crystal- 

 line planes, so as to build up one definite crystal, mechanically 

 complex, but optically and ci-ystographically simple, or merely 

 twinned. In a few special ca-es there is a well pronounced de- 

 parture from this rule, and truly compound groups of prisms are 

 formed. In the centre there is a definite simple prism, but 

 instead of this growing continuously in the same manner, so as 

 to produce a larger prism, its ends, a; it were, break up into 

 several smaller prisms slightly inclined to the axis of the first, 

 and these secondary prisms in like manner break up into still 

 smaller, so as ultimately to give riae to a curious complex, lirush- 

 like growth, showing in all positions a sort of fan-shaped 

 strncture, mechanically, optically, and crystographically complex. 



I have done my best to describe these various kind; of 

 crystals seen in blowpipe beads as clearly as can be done 

 without occupying too much time, but feel that it is impossible 

 to make the subject aa simple as it really is without numerous 

 illusti-ations. However, for the purpose now in view, it will I 

 trust suffice to have established the fact that we may divide the 

 •crystals in blowpipe beads into the following groups, which, on 

 the whole, are sufficiently distinct, though they necessarily pass 

 one into the other.- — 



1. Simple crystals. 



2. Minute detached needles. 



3. Fan-shaped compound groups. 



4. Feathery skeleton crystals. 



It must not be supposed that ciyst.als of one or other of these 

 groups occur promiscuously and w ithout some definite relation 

 to the special conditions of the case. Very much depends upon 

 their chemical composition. Some substances yield almost 

 exclusively those of one group, and other substances those of 

 another, whilst in some cases a difference in the rate of cooling 

 and other circumstances give rise to variations within certain 

 limits; and, if it were possible to still further vary some of the 

 conditions, these limits would probably be increa-ed. Thus, 

 for example, the earliest deposition of crystalline matter from 

 the glassy solvent is sometimes in the form of simple solid 

 prisms or needle-, but later on in the process it is in the form of 



compound feathery tufts ; and, if it were possible to cool the 

 beads much more slowly whilst they are very hot, I am inclined 

 to believe that some substances might be found that in the early 

 stage of the process would yield larger and more solid crystals 

 than those commonly met with. This supposition at all events 

 agrees with what take-; place when such salts as potassium 

 chloride are crystallised from solution in water. Some of my 

 blowpipe beads prove most conclusively that several perfectly 

 distinct crystalline substances may be contemporaneously de- 

 posited from a highly-heated vitreous solvent, which is an 

 important fact in connection with the structure of igneous rocks, 

 since some authors have asserted that more than one mineral 

 species cannot be formed by the slow cooling of a truly melted 

 rock. The great advantage of studying artificial blowpipe beads 

 is that we can so easily obtain a variety of results under con- 

 ditions which are perfectly well known and more or less com- 

 pletely under control. 



Artificial Slags. — I now proceed to consider tie structure of 

 slags, and feel tempted to enter into the consideration of the 

 various minerals found in them, which are more or less perfectly 

 identical with those characteristic of erupted rocks, but some of 

 the most interesting, like the felspars, occur in a well-marked 

 form only in special cases, where iron ores are smelted with 

 fluxes, seldom if ever employed in our own country, so that my 

 acquaintance with them is extremely small. My attention has 

 been mainly directed to the more common products of our blast 

 furnaces. On examining these, after having become perfectly 

 familiar with the structure of blowpipe beads, I could see at 

 once that they are very analogous, if not identical, in their struc- 

 ture. In both we have a glassy solvent, from which crystals 

 have been deposited ; only in one case tliis solvent was red hot 

 melted borax, and in the other glassy, melted stone. Thus, for 

 example, some compounds, like what I believe is Humboldtilite, 

 ciystallise out in well-marked solid crystals, like those seen 

 occasionally in blowpipe beads, whereas others crystallise out 

 in complex feathery skeletons, just like those so common in, 

 and ciiaracteristic of, the beads. In both we also often see 

 small detached needles scattered aboat in the glassy base. These 

 skeleton crystals and minute needles have been describedby 

 various writers under the names crystallites, bclonites, and trichites. 

 Though we have not the great variety of different forms met 

 with in the beads, and cannot so readily vary the conditions 

 under which they are produced, yet we can at all events see 

 clearly that their structural character depends both on their 

 chemical constitution and on the physical conditions under which 

 they have crystallised. None of my microspical preparations or 

 English slags appear to contain any speciei of felspar, but 

 several contain what I believe is some variety of augite, both in 

 the form of more or les; solid prisms, and of feathery skeletons 

 of great beauty and of much interest in connection with the 

 next class of products to which I shall call your attention, viz., 

 rocks artificially melted and slowly cooled. 



Rochs Artificially Melted.— \ have had the opportunity of 

 preparing excellent thin microscopical sections of some of the 

 results of the classic experiments of Sir James Hall. I have 

 also carefully studied the product obtained by fusing and slo.vly 

 cooling much larger masses of the basalt of Rowley, and have 

 compared its structure with that of the original rocks. Both 

 are entirely crystalline, and, as far as I can ascertain, both arc 

 mainly composed of the .same minerals. Those to which I 

 would especially call attention are a triclinic felspar and the 

 au-J-ite. The general character of the crystals is, how^ever, 

 strikingly different. In the artificial product a considerable part 

 of the augite occurs as flat, feathery plates, like those in furnac'^ 

 sla'Ts, which are quite ab-ent from the natural rock, and only 

 part 'occurs as simple solid crystals, analogous to those in the 

 rock, but much smaller and less developed. The felspar 1 ■ 

 chiefly in the form of elongated, flat, twinned prisms, which, 

 like the prisms in .some blowpipe beads, commence in a more 

 simple form and end in complex fan-shaped brushes, whereas 

 in the natural rock they are all larger than in the artificial, and 

 exclusively ol simple characters. On the whole then, though 

 the artificially melted and slowly cooled basalt is entirely 

 crystalline, and has a mineral composition closely like that of 

 the natural rock, its mechanical strncture is very different, being 

 identical with that of bljwpipe beads and slags. 



/'ciA-,!»;V vWfo.— Passing now to true natural igneous rocks, 

 we find some, like obsidian, which closely correspond w-ith 

 blowpipe beads, slags, and artificially melted rocks, in having a 

 crhssy base through which sm.all cryst-illine needles are scattered ; 



