f 
SS a ee 
C.—GEOLOGY. 69 
ground intrusions. The whole of the phenomena of the contact altera- 
tion of limestone, including the disappearance of original structures and 
organic remains, find a simple explanation through his experiments. 
Granted only a temperature about 1000°C. and sufficient pressure to 
retain the carbonic acid evolved (less than 1000 feet of average rock) 
any limestone will recrystallise completely. As a matter of fact, there 
is little evidence that the complete fusion of limestone is a common 
phenomenon, and a liquid limestone magma sending intrusive veins into 
the surrounding rocks has only seldom been postulated. The Huttonians 
thought that the calcareous amygdales of many of the basaltic lavas 
were fragments of limestone that had been involved in the igneous 
rock and completely fused, but this is no longer believed. Furthermore, 
Sir James Hall proved that under the same conditions limestone would 
react on silica, forming silicates, and would attack glass, porcelain, pipe- 
clay, and the material of his pyrometric cones; thus he explained the 
origin of accessory minerals of many limestones, such as wollastonite, 
garnet, vesuvianite, diopside, and scapolite. Edinburgh geologists, for 
example, know well the altered limestone which occurs at the margin of 
the teschenite-picrite sill at Davidson’s Mains railway station. There 
is no need to believe that it was ever completely melted, and the preser- 
vation of many traces of the original bedding makes it very improbable 
that complete fusion took place. 
Recrystallisation and the growth of crystals in solido were observed 
also by many of those who endeavoured to repeat Sir James Hall’s ex- 
periments during the nineteenth century, and have been fully confirmed 
by more recent researches. In fact this process is now regularly 
applied in the investigation of minerals that refuse to crystallise well 
from igneous melts or undergo transformation into other forms below 
a certain transition temperature. From the pure chemical components 
a glass is prepared as homogeneous and free from bubbles as possible, 
and this glass is then heated for many hours to a temperature below 
its melting-point, but within the field of stability of the crystalline form 
which it is desired to investigate. Crystals are thus produced which 
may be sufficiently large to have their optical characters, cleavage. 
hardness, and other properties satisfactorily determined. This is, of 
course, a case of devitrification, a process which Hall was familiar with. 
as he had studied it in the glass furnace at Leith which first suggested 
to him the advisability of making furnace experiments on rocks. He 
recognised it also in certain varieties of porcelain which he had employed 
in his experiments. But even when devitrification is sensiblv complete 
and a finely crystalline aggregate replaces the original glass the process 
will go on. and the crystals become larger and larger if subjected for a 
considerable time to a temperature not far below the fusion point. 
It was characteristic of Hall that having set himself an object 
he pursued it with undeviating persistence. For four years he con- 
tinued his experiments on the crystallisation of the carbonate of lime 
by heat modified by compression. In that time he made nearly five 
hundred experiments, and considering how elaborate they were and 
how all his apparatus was made by himself or by ordinary mechanics 
We cari see that little tithe was left for the ordinary pursuits of a country 
