(4) The structures and mutual relations of the rock masses. 



(5) The distribution of the different rock-types in space and time. 



As the present work will be devoted mainly to descriptive petrography, in 

 so far as this is dependent on the examination of hand specimens, (1) it will be 

 impossible to enter at length into the discussion of general questions relating 

 to the origin of rocks ; at the same time it will be equally impossible to avoid 

 all reference to such questions in consequence of their bearing on petro- 

 graphical classification. 



In considering the history of a rock it is necessary to bear in mind that 

 two more or less distinct questions are involved: (1) the origin of the minerals 

 of which the rock is composed, and (2) the origin of the rock itself. In some 

 cases there is the most intimate connection between the origin of the rock and 

 that of the minerals which enter into its composition : thus igneous rocks have, 

 for the most part, been formed by the successive crystallisation of the con- 

 stituent minerals from a molten magma. The minerals have been developed 

 in situ they are authiyenic. (2) In other cases, as for instance amongst 

 the sedimentary rocks, the origin of the minerals is for the most part wholly 

 distinct from that of the rock ; they have not been developed in situ they 

 are allothigenic. (8) 



Molecules are the units of the mineralogist ; minerals are the units of the 

 petrographer. Minerals are definite chemical compounds ; the laws which 

 determine their formation are those of chemistry ; the molecules of which 

 they are composed are brought together by the force of chemical attraction. 

 Rocks are for the most part mineral aggregates ; the laws which determine 

 their formation are those of physics ; the minerals of which they are com- 

 posed are brought together in various ways, but always under the influence, 

 and often by the direct operation of the force of gravity. (4) 



Consider now the causes by which rocks are known to be produced and 

 modified. In dealing with this subject a slight confusion arises in consequence 

 of the impossibility of separating by any hard and fast line the rock- 

 produciiig from the rock-modifying agents. The production of a rock is not 

 the result of one act, but of a long series of changes, and it must therefore be 

 remembered that if we regard any one of these changes as giving individuality 

 to the rock as marking so to speak its birth this is done merely for the sake 

 of convenience, and not for the purpose of limiting enquiry. It is impossible 

 to define with precision the moment in the history of a rock at which it 

 acquires individuality. A convenient classification, however, will be seen to 

 arise out of the following considerations. 



The rocks which form the surface of the earth are in contact with air 

 and water, and subject to various chemical and physical agents of change 



(1) This branch of the subject is frequently termed lithology. 



(2) Authigenic, from avQi t on the spot, here. 



(3) Allothigenic, from a \\odi, elsewhere, in another place. 



(4) See Lessen. Ueber die anforderungen der Geologic an die petrographische Systematik. 

 Jahr. d. geol. Landesanstalt, Berlin, 1884. 



