THE NATURAL HISTORY OF IGNEOUS 

 ROCKS: II. THEIR FORMS AND HABITS. 



TH E diverse forms assumed by bodies of igneous rock 

 are in a general sense familiar to every geologist ; 

 but our conceptions of them are to a great extent based 

 upon generalised and conjectural diagrams, and the subject 

 seems to be one worthy of closer inquiry. In the brief 

 review which follows we shall take note more especially 

 of those features which seem to have a bearing on genetic 

 questions, and the various considerations touched on below 

 will thus fall into place in connection with the former article 

 of this series. As we there remarked a close relation 

 between igneous activity and crust-movements, whether of 

 continent-building, plateau-building, or mountain-building 

 type, so we shall have to observe in what follows that 

 the forms and habits of igneous rock-masses are related to 

 the kind of crust-movement experienced by the region in 

 which they occur. Other factors are not without impor- 

 tant influence on the behaviour of igneous rock-masses, but 

 the connection of the latter with the geological structure 

 of the country is found to be of a very intimate character, 

 extending in many examples to special tectonic details. 



While this relation is more evidently verified in the case 

 of intrusive rocks, there are not wanting indications that 

 similar rules govern the phenomena of surface vulcanicity. 

 The association of volcanic foci and of fissure-eruptions 

 with the mountain-type and the plateau-type of structure, 

 respectively, can scarcely be regarded as a fortuitous co- 

 incidence. In makino- a few remarks on volcanic, before 

 passing on to intrusive, rocks we shall, however, draw at- 

 tention to some other points. 



The most obvious factors determining the forms assumed 

 by extruded lavas are the volume of an individual eruption 

 and the degree of fluidity possessed by the material when 

 poured forth ; and the latter must depend both on the 

 chemical composition of the lava and on the temperature 



