
; ; | af ; = F oe ; ? . 
556 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boor IV. 9 
In internal structure considerable differences may be detected 
among dykes. The rock may appear (a) with no definite structure 
of any kind beyond irregular jointing ; 

LOW -ware Kk : A 
KY FAL (b) columnar, the prisms striking off at 
SY right angles from the walls, and either 
vb, 
\» going completely across from side to 
side or leaving a central non-columnar 
part in which they branch and lose 
themselves: when the side of a dyke 
having this structure is laid bare, it 
presents a network of polygonal joints 
AS formed by the ends of the prisms which, 
Fic, 282.Puan or Dyke curring When the dyke is vertical, lie of course 
Sanpsrones, Suore, Gourock, in a horizontal position, whence they 
Sa depart in proportion as the dyke is 
inclined: occasionally the prisms are as well-formed as in any 
columnar bed of basalt; (¢) jointed parallel with the walls, the joints 
being sometimes so close as to cause the rock to appear as if it 
consisted of a series of vertical plates or strata: this platy cha- 
racter when it occurs in basalt dykes is best developed along the 
walls; (d) vesicular or amygdaloidal, lines of minute vesicles having 
been formed parallel with the walls, and attaining their greatest 
number and size along the centre of the dyke. 
As arule, the outer parts of a dyke of crystalline rock are finer- 
grained than the centre. Occasionally the external surface has a 
vitreous structure precisely analogous to that already described in the | 
case of intrusive sheets (p. 549). Basalt veins, for example, have not in- 
frequently an external coating or varnish (tachylite, hyalomelan, &c.). 
It occasionally happens also that the central portions of a dolerite 
dyke are glassy, of which structure several cases have been observed 
in Scotland ; perhaps in these instances the dyke has opened along its 
centre and a fresh uprise of more glassy basalt has risen in the 
fissure.’ 
Effects on Contiguous Rocks.—These are similar to the 
changes produced by intrusive sheets and other eruptive masses. 
Induration is the most frequent kind of alteration. Remarkable 
examples have been observed where, in limestones in contact with 
dykes, a saccharoid crystallization of the calcite has been super- 
induced, and where even new crystalline silicates have been 
developed (p. 572). 
Segregation Veins.—These include most of what were formerly 
and not very happily termed “contemporaneous veins,” and are 
peculiar to crystalline rocks, abounding in many granites, like- 
wise in some gneisses and schists, and not infrequently to be 
observed in sheets of diorite, dolerite, and diabase. They run as 
straight, curved, or branching ribands, seldom exceeding a foot in 
thickness. Sometimes they are finer in texture than the rock which 
’ See Proc, Roy. Phys. Soc, Edin, vol, v. 1880, p. 241. 
