1870.] MACPARLANE — ON CRYSTALLINE ROCKS. 51 



and the least objectionable among them would appear to be the 

 two last mentioned. The first of these, original (Urspriingliche,) 

 was first adopted by Zirkel* for denoting igneous or eruptive 

 rocks, while the term derivate was first suggested by David 

 Forbesj" as equivalent to secondary or sedimentary rocks. The 

 latter term we have ventured to modify, and in the following 

 pages we shall use the names original and derived for indicating 

 the two great classes. These names would seem to deserve the 

 preference, for the following reasons. It is admitted by geolo- 

 gists, on all hands, that the material which constitutes the various 

 sedimentary formations, consisting of limestone, hardened clay, or 

 consolidated sand, although it may have been immediately derived 

 from pre-existing rocks of a detrital nature, originally came from 

 the decomposition and disintegration of crystalline rocks, of such 

 as are known to constitute the oldest formations of the earth's 

 crust, or to have broken through and deposited themselves on the 

 outside of it. It is further an accepted theorem, universally 

 acknowledged by scientific men, that our globe was originally in a 

 state of igneous fusion, and that all the material which consti- 

 tutes the rocks of our day existed in the form of a melted zone 

 encircling the central part of the globe. It is evident that, before 

 the conditions for the formation of sedimentary rocks could exist, 

 the liquid globe must have become, to some extent, solid ; a crust, 

 at least, must have been formed upon it, from the disintegration 

 of which the material of such sedimentary rocks could have been 

 derived, and upon which that material could have been deposited. 

 This crust, and the rocks which from time to time after its 

 solidification penetrated or were erupted through it, must, conse- 

 quently, have been the first rocks, and they must have yielded the 

 material for all those subsequently formed by aqueous agencies. 

 It would, therefore, appear legitimate to name the former class 

 original, and the latter, derived rocks. 



Where, as in the case of the volcanic and sedimentary rocks 

 which are being formed at the present day, we can observe the 

 process of their formation, no doubt can arise as to their origin. 

 These rocks, however, form but a very minute fraction of those 

 which build up the earth's crust, and it becomes necessary, in 

 order properly to discriminate among the latter, to point out the 



* Petrographie I., p. 173. 



t The Microscope in Geology, p. 6. 



