422 Revieics — Marker's Igneous Hocks. 



folding, faulting, and overthrust; and "the cause of the displacement 

 of bodies of rock-magma is to be sought in readjustment of the Earth's 

 crust, which has become subjected to powerful stresses as a consequence 

 of deformation". 



These crust-movements are termed respectively the movements of 

 plateau-building and of mountain-building. In plateau-building there 

 may have been upheaval or depression of great blocks, and the volcanic 

 action manifested in connexion with them takes the form of fissure 

 eruptions. In mountain-building the main movements were due to 

 lateral thrust, accompanied by much folding and overthrusting, and 

 by what the author terms central eruptions. These eruptions are 

 regarded as due chiefly to relief of pressure in certain deep-seated 

 parts of the crust, where "such conditions of temperature and 

 pressure prevail that solid and liquid rock are in approximate 

 thermal equilibrium". 



In the case of fissure eruptions the outpouring may be due to " the 

 mere pressing outward of the fluid magma through fissures concurrently 

 opened, with a corresponding settling down of the heavier solid crust 

 which overlay the magma". Thus the Inner Hebrides form parts of 

 a sunken faulted area, on the margin of which igneous outpourings 

 took place. 



The old idea that eruptions may be caused by the penetration of 

 oceanic water to the region of the molten magmas is shown to be 

 untenable, the theory requiring us "to believe that steam can force 

 its way, against enormous pressure, through capillary channels in the 

 deeper parts of the Earth's crust ". On the other hand, water is 

 present in all igneous rocks, amounting "on the average to about 

 1^ per cent., a proportion quite sufficient to endow the molten rock 

 with the properties displayed in volcanic eruptions". The author is 

 rather disposed to consider, not that the sea is the source of volcanic 

 water, but that vulcanicity "is the original source of the oceanic 

 waters, and is slowly adding to them ". 



In giving, largely in his own words, a summary of his views on the 

 more generally interesting phenomena of igneous action, we have left 

 little space for any account of the author's more detailed descriptions 

 of fissure and central eruptions, and of the types of rock met with 

 in plateau-building and mountain-building — of sills and laccolites, of 

 phacolites, dykes, and batholites. In some cases, as pointed out, 

 laccolites may be the immediate cause of folding and faulting, in 

 others they appear as a consequence of displacements. 



The idea of certain large magma basins is supported by a study of 

 petrographical provinces, but the magma itself may undergo such 

 differentiation that drafts from it differ widely in composition. The 

 mutual relations of associated igneous rocks, the order of intrusion, 

 and the order of crystallization are all dealt with, as well as various 

 rock-structures and the effects of pneumatolytic action. The author 

 is opposed to the view that there is any great amount of assimilation 

 of bordering rocks by intrusive masses. 



The final chapter deals with the classification of igneous rocks, and 

 the author leaves to the future the establishment of a satisfactory 

 genetic classification. 



