CONCRETIONS AND SECRETIONS 125 
inwards. In this respect they differ from concretions which 
owe their origin, as already explained, to the aggregation of 
mineral matter round a central point, so that they grow from 
within outwards. Secretions are typically represented by the 
mineral matter which so often occupies the vapour cavities 
of ancient lava-form rocks. As these cavities are usually 
somewhat flattened from having been drawn out in the 
direction of flow, the subsequently introduced secretions are 
often almond-shaped. Hence they are termed amygdules, 
and the rock itself is said to be amygdalotdal. Such cavities 
vary in size from mere pores up to hollows measuring many 
inches in diameter. Sometimes the walls are lined with a 
mere film of mineral matter; in other cases the cavities may 
be largely or completely filled up (see Plate I.1). A secretion 
may consist of one and the same kind of mineral matter, or 
of successive bands of various minerals, and some of these 
bands may be distinctly crystalline, while others are crypto- 
crystalline or apparently amorphous. In other cases a cavity 
may be occupied by an irregular aggregate of different 
minerals—all more or less well crystallised. A hollow 
secretion, readily separable as a nodule from the rock in 
which it was formed, is termed a geode, while druse is the 
term applied to a cavity which is lined or studded with 
crystals. Nevertheless, “geode” and “druse” are sometimes 
used interchangeably. It is common, for example, to apply 
the term geode to siliceous secretions occurring in the form 
of hollow spheroids or balls, in such rocks as limestone and 
highly decomposed amygdaloids, from which the ball-like 
bodies are readily detached. . “Geode,’ therefore, refers 
rather to the secretion than to the cavity in which it occurs. 
“ Druse,” on the other hand, has reference not only to a par- 
ticular character of the secretion, but to the fact that it occu- 
pies a cavity. Hence, geologists often speak of drusy cavities, 
meaning by that simply crystal-lined hollow spaces. 
The secretions occurring in crystalline igneous rocks may 
be (a) original or synchronous, or (0) subsequent or superinduced. 
As types of the former (ovzgenal), may be cited the drusy 
cavities in granite (Plate X. 2), which are partially filled with 
well-crystallised examples of one or more of the original 
constituents of the rock. Obviously, such secretions must 
a 
