

STRUCTURE OF ERUPTIVE ROCKS 205 
rocks—the dyke along the line of junction becoming more or 
less markedly vitreous. Small vapour pores often appear at 
or near the margin, while larger pores, vacuoles, and occasion- 
ally irregular shaped cavities of some size occur towards the 
centre, either sporadically or forming a continuous medial 
zone running parallel to the direction of 
the dyke (Fig. 75). 
Dykes affect the contiguous rocks 
much in the same way as sheets, but to 
a less extent. In the case of dykes only 
a few feet in thickness, the alteration 
produced is very slight, but the broader 
dykes may bake and indurate the rocks 
for a yard or two away. 
Occasionally a dyke is the product 
of more than one intrusion—the same 
fissure having been rent open again and FG. 75.—DYKE, sHow- 
again so as to allow of successive injec- eas pata toes 
‘ : OF VAPOUR PORES 
memoee! tne same kind of molten matter np Vesicies. 
—the younger injections being often 
readily recognised by the “chilled edges” which they present 
to the rock they traverse. In other cases, however, the earlier 
and the later injections may be distinctly different—an erup- 
tion of basic rock having either preceded or succeeded one of 
acid rock. In such composite dykes a clear line of demarcation 
separates one injection from another. But in certain broad 
dykes of a composite structure, no such lines of separation are 
visible—one kind of rock gradually merging into another, so 
that the whole complex must obviously have been injected 
at or about the same time. The rock forming the sides of a 
dyke of this character is usually more basic than the central 
and larger portion. Near Liebenstein, in the Thuringian 
Forest, for example, there is a broad dyke, the flanks of which 
consist externally of melaphyre, which graduates inwards into 
syenite-porphyry, as this in turn merges into granite-porphyry, 
of which the central and major mass of the dyke is composed 
(see Fig. 76). Dykes of a like kind have been described by 
Professor A. C. Lawson as occurring in the Rainy Lake 
region of Canada—where in one and the same dyke the 
andesite of the marginal areas shades off inwards into a 

