1855.] FORBES FOLIATION OF ROCKS. 181 



pieces subjected to heat were, however, always more or less broken 

 up and friable, evidently from the unequal expansion and contraction 

 of the mass. To obviate this, it seemed necessary to have the auxi- 

 liary aid of pressure ; and, in order at the same time to make expe- 

 riments on a somewhat larger scale, I proceeded as follows. 



Having some small blast-furnaces at disposition, I placed slabs or 

 blocks of the rocks in question under the bottoms of them, which 

 were composed of a stamped mixture of clay and charcoal-dust, — and 

 which consequently protected them from the action of the air, whilst 

 at the same time they were exposed to a certain amount of pressure 

 from the slag and metal above them. This pressure varied as the sump 

 was filled with slag or metal, or a mixture of both, and as the former 

 was of average density 3, and the latter 5, and as the depth was 

 about 3 feet, we might have a pressure of from 7 to 12 lbs. per inch. 

 These experiments were made in Norway in a district where no great 

 variety of rocks were obtainable, and as they occupied a very long 

 time I can only here give the results of one or two, which, however, 

 are so far satisfactory. 



I exposed large slabs of an impure, rather micaceous clay-slate, 

 about 4 inches thick, 6 feet long, and 4 feet broad, under these cir- 

 cumstances, to the action of heat continued for some months, but 

 which never was so intense as to cause fusion or even softening, as 

 the slabs, even if they had been broken up, retained their original 

 outward form and sharp edges. From the pressure and the unequal 

 distribution of the superincumbent weight, they were very frequently, 

 if not ahvays, broken up, and consequently had to be removed, so that 

 generally some metal or slag is adherent to the pieces ; — also it some- 

 times happened, when little cracks had arisen in the protecting lining 

 of clay and charcoal, that the pieces would be found permeated to 

 great distances with small veins of metallic matter, so fine as to render 

 it astonishing how it could have occurred. In all cases, however, 

 where a softening had taken place, the result was spoiled, and the 

 foliated arrangement, which is otherwise so distinct, would be more 

 or less obliterated or totally destroyed. 



From the specimens now shown to the Society it will be seen that 

 the original slate is unrecognizable, and that we have a rock of a 

 white (probably feldspathic) ground, with specks of a black mineral 

 arranged with a distinct foliated structure. The appearances here 

 presented are strikingly like some of the rocks of the district, where 

 syenite comes in contact with mica-slate. 



The specimens, though not large, are, I think, quite characteristic. 



Again, on repeating these experiments with soapstone, using 

 blocks of this material (which is very abundant and is cut up with 

 axes and saws), I formed the bottom of a furnace of cubes of this 

 stone, 1 foot square, cemented by Stourbridge clay. After several 

 months, under precisely similar circumstances, the blocks retained 

 their external form and their sharp edges precisely as at first ; even 

 the axe-marks can be distinguished upon them. 



On fracture, however, it is seen that in parts they have all the 

 outward appearance of the chlorite found at Brevigstrand in Norway ; 



