56 



NATURAL SCIENCE 



[July 



works, from its superficial or microscopic aspect, the enveloping 

 crystals of pyroxene being described as ' large plates.' As all field- 

 workers know, they are often so uniform in length, breadth, and 

 thickness, as to produce the appearance of nodules on the weathered 

 surfaces of a rock. 



In dealing with columnar structure, it is remarked that in one type, 

 of which the rock at Fingal's Cave is an example, " the columns pass 

 with regularity and parallelism from the top to the bottom of a bed." 

 The other type is of the irregular character. " At Staffa the regularly 

 columnar bed is immediately overlain with one of the starch-like 

 character." Though we are told that the two types may even pass 

 into one another, we should have liked an opinion as to their con- 

 tinuity in the case of Staffa. In vol. ii., p. 210, the photograph of 

 that island is used to show the ' bedded ' structure of the basalt ; and 

 the reader might easily regard the mass as formed of two successive 

 sheets. Some reference would be useful to Scrope's observations in 



Diagram illustrating the gradual emergence of buried volcanic cones through the 

 influence of prolonged denudation. 



the Vivarais ("Considerations on Volcanoes," 1825, p. 141), already 

 noticed in Prof. Judd's ' Volcanoes.' 



Another case in which the opinion of Sir Archibald Geikie would 

 have been of service is in regard to the relation of laccolites to earth- 

 movement. How far does the pressure that propels the lava forward 

 enable it to lift up the dome of strata and to form a cistern for itself ? 

 In any case, we are not left with the vague idea that some expansive 

 force within the molten mass itself enables it to upheave the earth 

 above it (p. 87, and vol. ii. p. 361) ; but might not greater emphasis have 

 been laid upon the view stated on p. 98, where the cavities occupied 

 by the lava are themselves referred to earth-movement ? Mr Gilbert, 

 however, was undoubtedly of opinion in 1877 that the pressure on the 

 fluid mass was sufficient to produce the arching above the laccolite. 



A good summary of the theory of differentiation in plutonic masses, 

 and of recent results on ' bosses,' is given on pp. 88-99. The variation 

 of an intrusive magma by absorption of surrounding rocks is also very 

 fairly dealt with. 



Chapter vii. gives scope for some of those admirable descriptions 

 of landscape, which recur afterwards delightfully through the work, 

 and which constantly remind us of the open-air culture of the author. 



