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that we cannot resist the inference that we see there an offshoot from the 

 upheaving agent of which we are now treating. The term " Igneous rock, 

 however, is as likely to mislead as "volcanic action." Fire, as we commonly 

 understand the word, requires indispensably for its existence and support a free 

 communication with the atmosphere, and we must therefore largely extend its 

 meaning if we retain the term "Igneous rocks." The German geologists have 

 substituted the designation "Eruptiv-gesteine" to avoid this difficulty, and also 

 because the Metamorphic rocks, such as Gneiss, Clay-slate, &c., have doubtless 

 been subjected to heat as weU as those which we caU Igneous : but this term, too 

 is fax from perfect, for while shunning the error of an inexact specification of 

 the mode of formation, it involves an equal neglect of preciseness with respect 

 to the mode of appearance on the earth's surface, inasmuch as no distinction 

 is hereby drawn between the basalts and trachytes, which were ejected in a 

 fluid or viscous state, and the granites and porphyries, which were cooled and 

 solidified under intense pressure before their appearance. For the latter rocks, 

 usuaUy caUed Plutonic, Professor Jukes has suggested the epithet Irruptive, 

 or "Intrusive," restricting "Eruptive" to the analogues of the modern lavas. 



Although the Malvern rocks are largely metamorphic, that is sedimentary 

 deposits, which have assumed a crystalUne texture by heat, pressure, and o her 

 influences, which we are not yet able to comprehend, still we have abundant 

 opportunity to study on the hills and in the valley by Eastnor the mode of 

 occurrence and the countless varieties of these Igneous rocks. On the Clee 

 Hill also we see a typical member of this class, basalt, which has pierced through 

 and overflowed the Coal Measures. Some such rock as this has been the 

 upheaving agent of the Woolhope district, and the importance of this division 

 of the subject obHges us to consider three great questions connected with 

 Igneous rocks. What are they composed of ? Whence do they come ? How do 

 they cause these changes in the earth's surface ? Microscopic examination reveals 

 to us the minerals of which these rocks are composed as belonging to the 

 extensive class of SiUcates, and a chemical analysis resolves them into sihca and 

 various earthy and alkaline bases, alumina, potash, soda, Ume, and magnesia 

 with iron protoxide and sesquioxide. Why in some the bases, in others the sihca 

 should preponderate, has not yet been satisfactorily explained. To the question, 

 whence they come, the only possible answer is, from the earths interior; 

 but recent mathematical investigations have thrown considerable doubt upon 

 former theories about the state of matter there. M. Elie de Beaumont held 

 that "the whole globe, with the exception of a thin envelope, much thinner 

 in proportion than the shell to an egg, is a fused mass kept fluid by heat, but 

 constantly cooling and contracting its dimensions." Geologists generally have 

 maintained the same opinion : but from astronomical calculations, mto which we 

 will not now enter, Mr. Hopkins has proved that the earth's soUd crust, for which 

 a dark line is considered a sufficient representation in some manuals, cannot be 

 less than 800 or 1,000 miles thick. Professor Thomson has reasoned from 



