BEPOETS OF MEETINGS. 
325 
of this mineral, when examined under the microscope, exhibits a 
number of minute cavities filled by liquid, with the exception of 
a small portion occupied by a bubble, which is the cause of 
its milky appearance. On the application of heat, this vacant 
space above the liquid disappears ; and by this means Sorby 
attempted to determine the temperature at which the quartz 
began to solidify, and which seems to have been a little 
lower than that for granite. The main agents concerned in 
the deposition of quartz in such fissures have been reduction 
of temperature and pressure. In the granite and slate of 
Dartmoor we find that the latter originally formed a dome 
over the former, and has since been removed by the denuding 
forces of nature, the fissures having been formed after 
the deposition of the slate and the consolidation of the 
granite. Professor Sollas next proceeded to point out in 
what manner these fissures are connected by volcanic agency 
with the various mineral and metallic deposits, which occur in 
different portions of the earth’s crust. During the life of a 
volcano the shaft at its lower extremity terminates in a lake of 
molten rock. After a time the eruptions become less frequent, 
the molten matter solidifies into granite, and after a further 
interval thermal springs begin to appear, generally in the lines 
of fault. Devon and Cornwall were at one time covered with 
volcanoes, the remains of which exist in the form of bosses ; and, 
as Professor Judd has showm, the Western Isles of Scotland, and 
especially Mull, afford similar evidence. When fissures are 
formed near volcanoes, and extend downwards for several 
thousands of feet, being filled with heated water holding various 
matters in solution, it folio w’S that the solubility of these sub- 
stances, being dependent on temperature and pressure, as we 
ascend the fissures we should expect to find the different metallic 
compounds deposited according to their insolubility. Mr. John 
Arthur Phillips has devoted much time and attention to the 
