658 BEPORT— 1886. 



of course in tlie case of the earth's sphere there are many modifying circum- 

 stances, each aflecting the value of one or other of the factors of the formula 



n ' 



which represents the quantity (Q) of heat which passes through a homogeneous 

 plate of an area =A, a thickness =n, with opposite faces at constant temperatures 

 V.-, and ijj in the time (t), the coefficient of conductivity of the material being 

 represented by K. 



Among such modifying circumstances may be mentioned : — ■ 



(1) Variations in the material of rock-bodies, variations in the actual 



temperatures of the same rock, variations in the orientation of' the 

 crystalline architecture of many rocks, occurrence of mineral-veins and 

 lodes as good conductors, all affecting the value of K. 



(2) Variations in the capacity for heat of different rock-materials. 



(3) The unequal distribution of underground water. 



(4) The presence, in places, of air in the cavernous spaces of many rocks. 



(5) The mean temperature of the superficial strata as determined by latitude, 



together with surface-conditions (such as extensive forests, sandy 

 deserts, snowfields and glaciers, dry land or ocean bed), all aflecting the 

 value of the term v^ in the formula, in every case influencing, and in 

 some preventing altogether, radiation at the surface. 

 When all these conditions are considered there would still, however, appear to 

 be a considerable margin for increase, as deduced from the hypothetical case, even 

 though in the case of the earth the problem is still more complicated by the vary- 

 ing heat-producing causes in operation (pressure, crushing, friction, chemical action) 

 throughout the mass. 



2. The physical value which we attach to the terms ' weight ' and ' gravity ' 

 leads, with a little reflection, to the recognition of the fact that the same mass of 

 matter is not as heavy (and therefore exerts less pressure) within the mass of the 

 earth as at the exterior, since it is acted upon by the two opposite attractions of 

 the mass ' above ' and the mass ' below ' it. The latter diminishing, and the former 

 increasing with depth, the weight of the same mass of matter diminishes as it is 

 m_ade_ to approach the centre of gravity of the earth ; and when that coincides 

 with its own centre of gTa^ity the body is evidently altogether without weight. 



_ The determination of the curves which shall accurately represent the above two 

 principles requires the application of advanced mathematics ; but it is not difficult 

 to see that the curve of temperatures would have steep gradients for increase, and 

 the curve of pressures a less steep gradient for decrease, so that the two curves 

 (starting from the same origin) would intersect, and, in so doing, indicate realisable 

 conditions necessary for liquefaction at varying depths. 



9. xi Contrihiilion to the Discussion of Metamorplvisni in Rochs. 

 Bij the Rev. A. Ieving, B.Sc, B.A., F.G.S. 



The main object of this paper is to draw more attention than is usually given 

 to the physical and chemical side of metamorphic phenomena. As a step towards 

 the possible fixing of a more definite nomenclature for metamorphism, it is pro- 

 posed to exclude from it all such phenomena as the alterations which may arise 

 in the character of a rock from the removal in solution (as in dolomitisation of some 

 limestones), or the addition by infiltration (as in amygdaloids) of a mineral. 



All cases of true metamorphism may be included under the two heads of (i.) 

 Metatropy, (ii.) Paramorphism. 



(i.) Metatropy. — This term has been suggested by its sister-word ' allotropy,' which 

 is applied in chemistry to those modifications which one and the same chemical 

 (elementary) body may assume by changes in its jjhysical properties. Using 

 the word ' metatropy ' with rather more latitude, it is proposed to include under 

 this head such rock-changes as («) the conversion of a limestone into a crystalUne 



