TRANSACTIONS OF THE SECTION?. IH" 



rocks relative to their formation, more particulai-ly granite. While examining' 

 microscopically the various pitclistoue veins abounding in Arran, he was much 

 struck with the similarity of their structure, and the marked ditference they exhi- 

 bited when compared with sections of granite and its various mineral constituents. 

 On extending his observations to obsidian, marekanite (a volcanic glass from Lake 

 Marekan in Kamtschatka), and also to the well-known glassy obsidian of Bohemia, 

 he foimd they all exhibited a structure analogous to the pitchstones of AiTan. He 

 further foimd that sections of glass slags, where the heat had been long continued, 

 combined with slow cooling, all presented the same appearances as the sections of 

 pitchstone. 



This structure, peculiiir to igueously formed substances, ho found usually to radiate 

 in a stellate form ; and though many slags showed large stars visible to the naked 

 eye, the stellate structure is more easily observed by the aid of the microscope. 

 The character is so marked, that no one whose eye is tutored to microscopic obser- 

 vation can fail to recognize at once a mineral substance of igneous origin. 



In granite, on the other hand, the structiu-e, as seen by the microscope, is as per- 

 sistent as in pitchstone, glass, and obsidian, but totally different. 



In the many experiments which the author had tried with granites from various 

 localities, he had never succeeded in obtaining one instance of stellate structure, 

 while the constant occun-ence of cavities containing fluids convincedj him that, if 

 pitchstone and glass are types of igneous-formed substances, granite must be of 

 aqueous origin. In the fluid cavities so abundant in topaz, Caimgomm, beryl, 

 toiuTiialine, and felspar, all constituents of granite, he found the same appearance 

 prevailed. These cavities are seldom entirely filled with fluid, an air-bubble usu- 

 ally occupying more or less of the cavity. After many hundred experiments on 

 such cavities, the author found that when exposed to a temperature of 94° Fahr., 

 the bubble disappeared, the fluid entirely filling the cavity, and at the temperatm-e 

 of 84° the bubble re.ippeared with a sing-ular ebullition, showing that the .au' had 

 formed an atmosphere round the fluid. He was thus led to infer that these cavities 

 coidd not have been filled at a temperatm-e above 84°, and certainly not above 94° 

 of Fahrenheit. 



As anotlier proof that these cavities could not have been filled when the tempe- 

 ratm-e of the surroimding rock was higher than the temperature above indicated, 

 the author drew attention to the fact that the bubble of air occupied always a much 

 smaller portion of the cavity than the iluid, a condition which could not obtain, i:^ 

 as other wi'iters hold, the fluids were enclosed imder intense heat and pressure. 



For the purpose of accm-ately determining the temperatures at which the bubble 

 vanished and reappeared, the author constructed an apparatus which he exhibited 

 and described. It consists of a microscope with a hollow iron stage, having a tube 

 in the centi'e to admit light from the reflector. At one side, and inserted into the 

 stage, is a small tin retort with a stopper ; at the other side, a tube is inserted and 

 attached to a reservoir of water, fi-om which the hollow stage and retort are filled. 

 On applying heat to the retort by means of a spirit-lamp, any requhed temperature 

 under the boiliug-pomt of the water may be obtained in the stage and retort. 



Above the stage is placed an iron saucer, in the centre of which an iron tube is 

 riveted, thi'ough which the light is admitted ; this vessel is filled with mercury, and 

 in it is placed an upright thermometer, with the bulb shielded with cork or any other 

 good non-conductor ; by this means it indicates the actual temperature of the mer- 

 cury bath. The cavity to be observed is cemented with Canada balsam to a plate 

 of glass 3x1 inch, and is floated on the smface of the mercury, so that the glass 

 and mercmy are in absolute contact. When the temperature is raised imtil the 

 bubble nearly disappears (which is seen by its contraction), the spirit-lamp is with- 

 drawn, aud the vanishing point carefully watched and the temperatiu'e noted. The 

 stopper of the retort is then withdrawn, and the stopcock of the reservoir of water 

 opened, so that the temperature of the stage and mercm-y bath is soon reduced, and 

 the ebullition or reappearance of the bubble takes place, when the temperature is 

 again recorded. By this method the author felt confldent that his results were 

 con-ect, as they always were consistent when observing the same cavity. By means 

 of this instrument the author had found fluid cavities in the trap tutf of Arthur's 

 Seat, the greenstone of the Crags, and the basalt of Samson's Ribs. He had also 

 foimd that the porphyry of Duu Dhu in Arran, which jnost geologists asaime^ 03 



