Reviews — Harker's Igneous Rocks. 421 



problems of petrology are in great measure independent of the 

 speculations of cosmogony." 



We have, therefore, a suitable starting-point in the data furnished 

 by geological observation, data which from an historical point of view 

 are supplied more fully in the British Isles than elsewhere. The 

 geological record indicates that there were great periods of igneous 

 activity, separated by intervals of quiescence ; but that there is no 

 evidence of a decline of igneous energy, nor of any differences in the 

 types of intrusive or extrusive rocks in different ages. In most cases, 

 where clear evidence has been obtained, the sequence of events has 

 been (1) volcanic or extrusive, (2) large plutonic intrusions, (3) minor 

 intrusions. 



The indiscriminate use by some writers of the terms ' vulcanicity ' 

 and 'volcanic rocks' for igneous phenomena in general, is deprecated 

 by the author. The superficial outbursts cannot be regarded as the 

 most important results of igneous action unless from a present-day and 

 personal point of view. Despite the enormous outpourings of lava 

 that have taken place in some regions, it is pointed out that intruded 

 rock-masses enter mucli more largely into the outer crust of the earth, 

 and there is more to be learnt from the records displayed in eroded 

 regions of former igneous activity than from a contemplation of active 

 volcanoes. An analogous process is in many respects applicable to the 

 study of glacial phenomena. In the case of igneous rocks, however, 

 "Instead of applying a knowledge of processes now going on around 

 us to elucidate the record of past ages, we must seek rather to use the 

 history of the past to explain the phenomena of the present." 



As the author remarks, "Igneous action, in its dynamic aspect, 

 consists in the moving of a body of magma from one situation to 

 another, in response to differences of fluid pressure ; but whether or 

 not some part of the magma is forced out at the surface depends upon 

 conditions of the second order of importance." 



Without discussing at any length the origin of igneous magmas, the 

 author points out that modern volcanoes negative the idea that they 

 draw directhj upon a common source or 'stock-reservoir', as evidenced 

 b}^ differences in the levels of lava columns in adjacent areas and in 

 the composition of ejected materials. Nevertheless, community of 

 relationship and of origin is met with in some cases, indicating an 

 extensive magma-basin or series of basins. The ascertained facts lead 

 to the conclusion that there are reservoirs of molten magma of different 

 magnitudes and permanence, and that when exhausted by eruption 

 or solidified, subsequent outbursts may be caused by re-melting or 

 by fresh accessions of molten material from deeper sources. Indeed, 

 "the largest igneous rock -masses known to geologists may represent 

 subsidiary reservoirs, which were only off-shoots from the main basin 

 and at a higher level. In confirmation of this, it is found that such 

 masses very generally give evidence of distinctly intrusive relations, 

 and are sharply separable from the contiguous rocks, which, though 

 highly metamorphosed, show no sign of fusion." 



Geological evidence proves that there has been a general, but not 

 invariable, correspondence between igneous action and important move- 

 ments in the earth's crust, such as those of elevation and depression. 



