490 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
at the white heat it fused to a blebby scoriaceous slag, of a fawn colour, very 
similar in appearance to some vesicular lavas. 
The data presented to us by the first experiments were that the shales had 
lost all their combustible ingredients, while the Lydian stone had not been 
deprived of any of its water. We have ascertained that the shales are totally 
decomposed at the temperature of boiling mercury, while the Lydian stone is 
partially decomposed at a heat below redness ; we are therefore in a position to 
say that—though the restraining effect of pressure may have counteracted the 
decomposing power of a higher temperature at great depths,—the temperature 
at which the ashes were jinally ejected from the volcanic vents, which ruptured 
the Lower Coal strata, probably lay between 660° and 900° Fahr. 
Let us now see what data we have to work upon, in attempting to estimate 
the temperatures at which ashes, cinders, mud, and fragments of penetrated 
rocks were ejected in Old Red Sandstone days. 
As regards the augitic masses described, we have two circumstances to 
found upon: the first, that the included felspar—clearly one or other of the 
soda felspars, probably labradorite—had been so perfectly liquefied that it 
had assumed spherical forms ; a fact which also necessitates at least a viscous 
condition in those portions of the augite which included the felspathic spheres ; 
the second, that we have, in the old cleavages and unaltered portions of the 
augitic masses, evidence either that the heat had not been high enough to 
vitrify them throughout, or that the period of time during which they had been 
subjected to that heat had not been sufficiently extended to enable them to be 
uniformly altered. In other words, the heat had been such as thoroughly to 
liquefy the felspar, but not such as thoroughly to liquefy masses of the augite. 
What heat then had sufficed ? 
The process employed in the determination of the water in labradorites had 
shown that mineral, and the plagioclastic felspars generally, to be liquefied at 
the temperature of a full, but hardly a bright yellow heat.* 
The point of liquefaction of the augites varies largely,—from readily fusible 
varieties, to the almost infusible diallage. 
Direct experiments on the mineral in question were therefore necessary. 
Preliminary trials in the blowpipe showed that ordinary-sized chips of the 
mineral were only rounded on the edges, or fused with difficulty in the best 
flame that could be obtained ; but when subjected to FLETcHER’s blowpipe, in 
which the air and gas are both passed through red-hot tubes before combustion, a 
perfect glass, of a colour somewhat paler than the mineral, was readily obtained. 
A quantity, in chips and powder, was heated at gradually increasing tem- 
peratures in a platinum crucible ; it was found that the highest heat of a fine 
* Orthoclase requires nearly a white heat. 

