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No. 3.] MACFARLANE—ON CRYSTALLINE ROCKS. 279 



loss, those rocks will be found at the head of the list )s-hich are 

 commonly considered to be the oldest in age. Delesse found the 

 following per centages of diminution, the specific gravity of the 

 various rocks before fusion being regarded as = 100. 



Granite, graniilite and quartz porphyry 9 — 11 p. c. 



Syenitic granite, and syenite 8 — 9 " 



Porphyry with orthoclase and oligoclase, with 



and without quartz 8 — 10 



Diorite and diorite porphyry 6 — 8 



Melaphyre 5 — 7 



Basalt, tracliyte, and old volcanic rocks 3 — 5 '? 



Lavas and volcanic rocks — 4 " 



As early as 18-41, Gustav Bischof made observations on the 

 comparative volumes of Basalt, Trachyte and Granite in their 

 crystalline, melted, and vitreous conditions, with the following 

 results : 



Volume in vitreous condition. in crj'stalline. 



Basalt 1 0.9298 



Trachyte 1 0.9?14 



Granite . , . . 1 8420 



Volume in a fluid state. , in crystalline. 



Basalt 1 .' 0.8960 



Trachyte 1 0.8187 



Granite 1 ,. 0.7431 



Nothing can be more obvious from these data and experiments 

 than that original rocks in cooling, solidifying and crystallising, 

 underwent contraction, increasing thereby their density, and that 

 the amount of contraction was the greater the more thoroughly 

 und coarsely crystalline the rock, and the earlier the dates of its 

 eruption in the geological history of the earth. It is not custo- 

 mary in treating of eruptive rocks usually to entertain any very 

 definite ideas as to their age, but it ought not to be forgotten 

 that the geological experience of Europe has shewn that they 

 made their appearance on the earth's surface somewhat in the 

 game order as they occupy in Table III. It would therefore 

 eeem that those rocks which have experienced most perfect crys- 

 tallisation and the greatest amount of contraction or increase of 

 density during that process are the oldest in geological age, that 

 those which have crystallised imperfectly and experienced but a 

 moderate amount of contraction, belong to the middle age of 

 geological history, and that those which have solidified quickly 

 to a semi-vitreous condition, and have experienced in so doing 

 scarcely any contraction, are exactly those which are the most 

 recent, and have been denominated volcanic rocks. Such results 

 ought not to surprise us, but ought rather to be anticipated if 



