CXXU PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



a temperature of 248°, loses its water ; and, if heated to a much 

 higher temperature, does not regain it ; so that it may be reasonably 

 assumed that contact with an igneous rock of intrusion would 

 change gypsum into anhydrite. 



The action of heat upon siliceous rocks is somewhat different in 

 character, and had been the subject of a previous essay of M. Delesse. 

 Their fusibility diminishes with the increased amount of their silica ; 

 and they fuse either into a glass or into a scoria more or less 

 crystalline, at the same time being diminished in density, and after 

 fusion being readily acted upon by alkalies. 



As prismatic sandstones are often found in contact with trap- 

 dykes, M. Delesse has tried what the effect of the action of alkalies 

 would be on a sandstone forming the lining of a furnace, and which 

 had assumed a prismatic condition as the effect of heat. This sand- 

 stone was very quartzose and cellular, or melted only in some few 

 points. When boiled in a concentrated solution of potash, 4*5 of the 

 silica was extracted, — this experiment proving that, by the joint 

 action of heat and the alkaline constituents of an igneous intru- 

 sive rock, the metamorphic effects would be considerable, as the 

 rock is brought by the heat into a condition in which it becomes 

 subject to the alkaline action. 



It is manifest that hitherto M. Delesse has estimated the effects 

 produced under the simplest aspect of the operations of natural 

 causes ; but, as in Nature the causes are complicated or combined (for 

 example, in volcanos, where heat acts in the interior of the earth 

 subject to the modifications due to pressure and the action of 

 vapour and disengaged gases), he then proceeds to give examples of 

 the alterations both of minerals and of rocks by subterranean igni- 

 tion such as often occurs in coal-mines. The heat thus produced, 

 and under such circumstances, has been found sufficient to volatilize 

 the water, and disengage the carbonic acid of the argillaceous 

 carbonate of iron, and to produce a considerable change in its 

 physical characters, whilst combustible substances have also undergone 

 material alteration in such a process. In this latter case, the carbon 

 being dissipated, the ashes are sometimes too refractory to melt, as 

 at Menat in Auvergne, where the ash proceeding from the spon- 

 taneous combustion of a bed of lignite is a schistose and pulverulent 

 tripoli coloured red by oxide of iron, whilst in other cases they have 

 melted and are vitrified. Considerable change may be effected also 

 even where the combustibles are not burnt, as the observations of 

 M. Drian, quoted by M. Delesse, prove — on such natural combustions 

 at the Montagne de Feu near Lyons. The first alteration is indi- 

 cated by the coal becoming iridescent ; by a further change it 

 becomes cellular and cavernous, as well as harder and more brilliant, 

 and finally passes into a coke with metallic lustre : and such altera- 

 tions have been extended to a distance of several yards from the 

 ignited coal, the metamorphosis here produced being analogous to 

 that observed in contact with trap-rocks. Siliceous rocks are acted 

 upon nearly in the same manner as by ordinary calcination : they 

 lose their water or other volatile constituents ; they become, if argil- 



