314 
than other magmas of corresponding acidity. Thus nepheline- 
syenites often consolidate at a temperature lower than the 
syenites with which they are associated. It will be seen, there- 
fore, that the usual order of succession of associated abyssal 
rocks — (1) gabbro, (2) syenite, (3) nepheline-syenite, and 
(4) granite — corresponds better with a law of decreasing temp- 
erature of consolidation than it does with a law of decreasing 
basicity. 
For the closer study of the influence of the temperature 
of consolidation upon the relative age of associated abyssal 
rocks, we may consider some important cases separately. For 
the syenitic and granitic rock-bodies here discussed, the usual 
batholitic habit will be assumed and it will easily be seen that 
many of the results are also applicable to large laccolites. 
Gabbro and syenite (or granite). — It can scarcely be 
doubted that large syenitic (and granitic) batholites may often 
remain more or less fluid during the whole period of intense 
igneous activity, and though intrusion may have commenced at 
an early date (p. 312), consolidation can only be expected when 
the igneous activity begins to decline. Even small batholites 
must remain fluid for a very long time. If we assume that a 
gabbroid magma is intruded into the upper earth crust during 
the same period of intense igneous activity, but later than a 
syenitic magma, the gabbro will probably not break through the 
unconsolidated syenite, but the magmas may perhaps come into 
contact somewhere along a more or less vertical surface along 
which, by reason of the viscousity of the syenite, they may not 
mix. In this case, as mentioned above, contact relations will 
entirely depend upon the order of consolidation; the syenitic 
magma, solidifying at the lowest temperature, in some places 
may send out apophyses into the gabbro; in other places it 
may encroach upon the gabbro by magmatic stoping and, if so, 
it will become fine-grained at the contact; in other places per- 
haps no signs of difference of age will be visible. Such in 
