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many places are the actually observed contact relations between 
associated gabbros and syenites, and it may be concluded that 
these relations, while justifying us in saying that the syenite is 
younger than the gabbro, do not settle the question of the 
original order of intrusion, and still less the order of their dif- 
ferentiation. 
Contact relations which, on the other hand, show syenite 
younger than gabbro may, however, throw light upon the 
question of the sequence of intrusion. Thus, when syenite is 
overlain directly by a gabbro of earlier consolidation, we may 
with certainty conclude that the intrusion of the syenite is 
later than the consolidation of the gabbro. In this case since 
a heavy magma cannot rest upon a lighter one, the gabbro 
cannot have been fluid when in contact with the syenite. 
Nevertheless it may be quite doubtful whether or not the 
observed sequence agrees with the original sequence of intru- 
sion. Syenite may be extremely slow in stoping its way through 
the overlying rocks, and may perhaps meet with solid gabbro 
sills or laccolites which have been intruded by a process of 
relatively rapid intrusion at a time later than that at which the 
syenitic magma began to invade the upper earth crust. 
When the igneous activity decreases, the batholites will 
slowly consolidate. When they have done so, the igneous ac- 
tivity will often be near its final wane. Batholites are, there- 
fore, in many cases traversed by but few independent dykes, 
and as basic dykes are the commonest of all dykes, the latest 
of these will probably be basic. 
If after a long interruption igneous activity recommences 
the batholites will have attained a solid state. Syenites under 
these circumstances may be invaded by a gabbroid magma, 
and exceptions to the ‘rule of decreasing basicity’ will result. 
In such exceptions to the rule, the observed order of consoli- 
dation has a real bearing upon the original order of sequence 
of intrusion. 
