568 . REPORT— 1880. 



blowpipe teads is that we can so easily obtain a variety of results under conditions 

 which are perfectly well known, and more or less completely under control. 



Artijici(tl Slags. 



I now proceed to consider the 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 structure. In both we have a glassy solvent, from which 

 crystals have been deposited ; only in one case this solvent was red hot, melted 

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

 like what I believe is Humboldtilite, crystallise out in well-marked solid crys- 

 tals, like those seen occasionally in blowpipe beads, whereas others crystallise 

 out in complex feathery skeletons, just like those so common in and characteristic 

 pf the beads. In both we also often see small detached needles, scattered about 

 in the glassy base. These skeleton crystals and minute needles have been de- 

 scribed by various writers, under the names, crystallites, belonites, and trickites. 

 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 microscopical preparations of English slags appear to 

 contain any species of felspar, but several contain what I believe is some variety of 

 augite, both in the form of more or less 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. 



Hocks artificially melted. 



I 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 slowly cooling much larger 

 masses of the basalt of Rowley, and have compared its structure with that of the 

 original rock. Both are entirely crystalline, and, as far as I can ascertain, both 

 are mainly composed of the same minerals. Those to which I would especially 

 call attention are a triclinic felspar and the augite. The general character of the 

 crystals is, however, strikingly different. In the artificial product a considerable 

 part of the augite occurs as flat, feathery plates, like those in furnace slags, which 

 are quite absent 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 is 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 larger than in 

 the artificial, and exclusively of simple character. 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 structure is very 

 different, being identical with that of blowpipe beads and slags. 



Volcanic Hocks. 



Passing now to true natural igneous rocks, we find some, like obsidian, which 

 closely correspond with blowpipe beads, slags, and artificially melted rocks, in 

 having a glassy base, through which small crystalline needles are scattered ; but 

 the more completely crystalline volcanic rocks have, on the whole, a structure 



