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. 
