
208 . STRUCTURAL AND FIELD GEOLOGY 
coarse-grained autogenous veins is shown by the fact that they are not 
always sharply separated from the rock on either side, as is the case with 
exogenous or intrusive veins. On the contrary, the mineral constituents 
ef an autogenous vein often interosculate, as it were, with those of the 
surrounding granite—the crystals of the latter being so interlocked with 
those of the vein, that the two rocks are not readily separated along their 
line of junction. 
Contemporaneous veins are met with in many other eruptive rocks, 
more particularly in batholiths and thick sills of such rocks as gabbro, 
dolerite, and diorite. 
Yet another kind of autogenous veins may be mentioned. These 
are the so-called segregation veins. They are distinguished from the 
other varieties described by the fact that they merge gradually into the 
enclosing rock of which, therefore, they are merely a coarsely-crystalline 
modification. They have not been injected into rents or fissures after 
the manner of other endogenous veins, but their precise mode of origin 
is obscure. They appear to be the result of some process of segregation, 
and to represent zones or lines along which crystallisation of the con- 
stituent minerals was more readily developed than elsewhere in the 
same rock-mass. Although of common occurrence in eruptive rocks, 
segregation-veins are not confined to these, but make their appearance 
also in certain schists, and even in derivative rocks which have been 
more or less metamorphosed. 
BFFUSIVE ERUPTIVE ROCKS 
Effusive rocks have been erupted at the earth’s surface, 
and are of two types, crystalline and fragmental—that is to 
say, /avas and ¢uffs. As they frequently occur interstratified 
in a conformable manner with derivative rocks of all kinds, 
they are often termed contemporaneous or interbedded. 
(2) Crystalline Effusive Rocks.—The general petro- 
graphical characters of these rocks have been already set 
forth. It will be remembered that lavas are often scoriaceous 
above and below, and in some cases may be more or less 
porous and cavernous throughout. The vapour-cavities are 
often flattened or drawn-out in the direction of flow. In all 
such lava-form rocks residual glassy matter is very commonly 
present, especially towards the upper and under surfaces, 
solutions might send out the very finest threads and veins into the 
contiguous rocks, while other portions would collect as geodes and veins 
in the interior of the magmatic mass. As the solution cooled, one 
substance after another would separate out—and if the cooling process 
were not too rapid, the minerals would segregate in large crystals, such as 
characterise the pegmatite-veins. 

