TEANSACTIONS OF SECTION C. 659 



marble, (6) the conversion of grauwacke into hornstone or porcellanite, (c) the 

 conversion of seam-coal into anthracite, and that into graphite, (d) the conversion 

 of anhydrite into gypsum, together with some other cases in which minerals con- 

 stituting rock-masses (a) acquire a crystalline form by taking up water, or (/3) have 

 their crystalline form altered (polymorphism) by a change in their proportion of 

 water of crystallisation or by crj-stallisation at different temperatures {e.ff., car- 

 bonates of lime and magnesia). ' Devitrification ' of glassy rocks (e.ff., tachylite, 

 obsidian) — at once the most interesting and most obscure of metatropic changes — 

 is believed by the author, from the consideration of a large number of chemical facts 

 known in connection vdth the modifications of such bodies as sulphur, phosphorus, 

 arsenic, silica, borax, metaphosphoric acid, sodium metaphosphate (all of which 

 occur in the glassy or vitreous modification) to consist essentially of a chmiffe in 

 molecular structure. Further this change appears to proceed, in all cases, by the 

 gradual building-up of more stable and more complex molecules out of the less 

 stable and less complex molecules, of which the body appears to be made up in 

 the vitreous condition ; and this change the author conceives to be connected with 

 the loss of latent heat of vitrification, to which a large number of facts connected 

 with the behaviour of ithe above-named bodies, as well as those arrived at from 

 recent researches into the properties of artificial glasses, seem to point. M. 

 Daubree's researches are referred to as showing how heat and pressure combined 

 may, by increasing enormously the solvent action of water, aid in the process. 



(ii.) Paramorphism. — A term to include all those instances of metamorphism 

 which consist essentially of the decomposition to any extent of the original mineral- 

 constituents of a rock with the production of new minerals of a difl'erent chemical 

 composition within the rock. Such changes may take place in two ways ; (1) in 

 the dry way, when reactions between contiguous minerals are set up by their being 

 brought into a state of fusion •, (2) in the wet way (probably by far the more 

 common) from the presence of interstitial water, acting, at high temperatures and 

 pressures (a) as a direct solvent, (6) indirectly by the transfer of mineral matter 

 by percolation through the rock-mass, a process immensely facilitated by crushing, 

 and favoured by the easy access of water along junction-planes ; the true explana- 

 tion, perhaps, of many of the known instances of («) the development of new 

 minerals in ' contact metamorphism ' {e.y., in the hornstones of the Hartz and in 

 the Triassic limestones of the Predazzo region), {b) the apparent transition of true 

 crystalline schists into granite. Such a theory is a very diflerent thing from the 

 ' hydvochemical theory ' of Bischof and of those who have followed him. 



Chemistry, and especially thermal chemistry, enables us to make such dediic- 

 tions from tlie nebular hypothesis as to throw great light upon the order of succession 

 in the kinds of deposits of which the earth's crust has been built up, and points to 

 the first oxidation of hydrogen at very high temperatures, but under an enormous 

 atmospheric pressure, as a fact which must have occurred at a very early stage ; 

 and this the author regards as the main factor in the development of those primary 

 characters of the Archsean gneisses and schists (though other characters have been 

 subsequently induced) which distinguish them, on the one hand from the highly 

 siliceous materials which were previously concentrated in a state of dry fusion, and 

 on the other from all the later formations, beginning with the Silurian, which have 

 the character of ordinary aqueous deposits (though often subsequently metamor- 

 phosed in various ways and degrees) in true aqueous basins, and under atmospheric 

 conditions approximating to those which prevail at the present time. The occur- 

 rence within the crust of conditions, effectively the same as those which are regarded 

 as having been universal at the earth's surface in Archaean times, as the result of the 

 combined action of pressure, crushing, friction (shearing and sliding), and super- 

 heated water, is fully recognised ; but these are regarded as exceptional local develop- 

 ments of the conditions necessary to produce the highest degrees of metamorphism. 

 Tangential pressure, arising from contraction of the crust, is regarded as the 

 primary cause of these local developments, as well as of less marked phases of 

 * regional metamorphism' of later periods. The frequent and extensive unconformity 

 observable between the Archaean and the Silurian (and younger) formations, as 

 well as the occurrence in them (down even to the Cambrian) of rolled and worn 



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