THE 



GEOLOGICAL MAGAZINE. 



No. LXVIL— JANUARY, 1870. 



L — On the Contraction of Igneous Eooks in Cooling. 

 By David Forbes, F.R.S., &c. 



UPON commencing, in 184:7, the special study of the igneous rocks, 

 I, no doubt in common with many others, took for granted the 

 conclusions arrived at experimentally by Professor Bischof, of Bonn, 

 as to the amount of contraction which such rocks would undergo 

 when passing from the state of hot liquidity to that of cold solidity ; 

 and I did so, not after any critical examination of the details of 

 Bischof's inquiry itself (the most important of which are contained 

 in a memoir published inLeonhard und Bronn's Jahrbuch for 1841), 

 but merely in deference to authority, since these conclusions had 

 apparently received unquestioned acceptation from geologists in 

 general. 



Living at that time in Norway, a country pre-eminently fitted for 

 the study of eruptive rocks on the large scale, I soon found, however, 

 that these conclusions did not seem to be at all in accordance with 

 the facts which every day presented themselves in the field. I could 

 nowhere find evidence to prove that any such immense contraction 

 as 25 per cent., or one-fourth of its volume in granite, or 10 per cent., 

 one-tenth of its volume in basalt, as assumed by Bischof, had ever 

 taken place in nature. Even when great masses of granite were 

 seen interposed between rigid rocks of other character, they were not 

 found to present large fissures or rents of contraction, or to possess 

 a well-marked porous, honeycombed, or cavernous structure, such as 

 might be expected in case any such enormous contraction in volume 

 had really taken place. 



On the contrary, although some small amount of contraction had 

 undoubtedly taken place, these rocks, as a rule, appeared to be so 

 solid and compact in texture throughout, as to indicate that the space 

 which had been originally filled up by molten rock differed but com- 

 paratively little in volume from that which the rock now occupies. 



These discrepancies led me to consult the original memoirs of 



Bischof, and to repeat his experiments on a more extended, and, in 



some cases, a much larger scale, using as materials the basaltic mela- 



phyre of Staffordshire, commonly called the Eowley Eagstone, 



VOL. VII. — NO. rxvri. 1 



