"Vol. 63.~] CONSTANTS IN MINERALS AND IGNEOUS ROCKS. 



151 



expansion that the molten glass completely filled the loop and 

 welled out, flooding the platinum-ribbon on each side. 



Name. 



Locality. 



Sp. Gr. of 



1 Crystalline 



Rock. 



1 'Granite Shap Fells (Cumber- 

 land). 

 Peterhead (Aberdeen) 



2 | Granite 



3 Sj r enite 



4 iTonalite 



5 iDiorite 



6 IDiorite 



7 Gabbro 



Plauen'scber Grand, 



Dresden. 

 New Zealand 



Guernsey 



8 JQuartz-enstatite - 



diabase. 



9 tRhyolite 



Olivine-dolerite ... 



Dolerite 



Dolerite jWhin Sill.... 



New Zealand 



Markfield (Leicester, 

 shire). 



Carrock Fell (Cum- 

 berland). 



Penmaenmawr 



Tardree (Antrim) .. 



Clee Hills (Shrop- 

 shire). 

 Rowley Rag 



13 ;Micropoecilitic 

 1 andesite. 



14 jMicropcecilitic [New Zealand 



andesite. 



15 



Hyalopilitic an - New Zealand 

 desite. 



2-656 

 2-630 

 2-724 

 2-765 

 2-833 

 2-880 

 2-940 

 2-790 

 2-460 

 2-889 

 2-800 

 2-925 

 2-670 

 2*700 

 2-692 



Sp. Gr. 1 Change 



of of 



Glass. Sp. Gr. 



2-446 

 2-376 

 2-560 

 2-575 

 2-680 

 2*710 

 2-791 

 2-578 

 2375 

 2-775 

 2-640 

 2-800 

 2525 

 2-570 

 2-550 



210 

 254 

 164 

 190 

 153 

 170 

 149 

 212 

 085 

 114 

 160 

 125 

 145 

 130 

 142 



Percentage 

 Increase 

 in Volume. 



8-58 

 10-69 

 6-40 

 7-37 

 5-70 

 6-27 

 5-41 

 8-20 

 3-50 

 4-14 

 6-06 

 4-46 

 5-74 

 5-05 

 5-56 



If a mass of igneous rock be imagined in the earth's interior, 

 kept solid by pressure, though far above its melting-point : on a 

 sudden release of this pressure, by a dislocation along a fault or 

 flaw-plane, the conditions would be analogous to the above, and an 

 immense outpouring of lava might take place. 



The melting-points of the glasses formed by the fusion of the 

 above rocks were taken by means of a meldometer, constructed 

 after the model of that invented by Prof. Joly (6). The tempe- 

 ratures are calculated from the observed expansion of a platinum- 

 ribbon, previously calibrated with substances of known melting- 

 points, which takes place when it is heated by an electric current. 

 A description of this instrument, with its method of use, is given 

 by Mr. E-. S. Cusack (2). The substances used in calibration of the 

 ribbon were silver-chloride, potassium-bromide, potassium-carbonate, 

 copper-oxide, and palladium. The current was the same as that used 

 for the platinum-furnace, but it was brought down to a suitable 

 strength by means of galvanized-iron wire resistance-coils. There 

 was also an additional rheostat, in the form of two parallel German- 

 silver wires connected by a sliding bridge : this had a total resist- 



