MACROSCOPIO STEUCTUBE 31 



some of wHeli are common to all of the primary groups above 

 mentioned, while others are limited to one or more. Rocks 

 which are made up of distinct grains, whether crystalline or 

 fragmental, are spoken of as granular; when the structure be- 

 comes too fine and dense for macroscopic determination it is 

 spoken of as compacty though there is no reason why the term 

 should not equally well be applied to the coarser grained rocks 

 in which the individual grains are closely cohering without 

 interstices. The term massive is applied to such igneous rocks 

 as show no signs of bedding or stratification, while limestones, 

 sandstones, and such other rocks as are arranged in more or 

 less parallel layers are described as stratified. (See Pig. 1, 

 PL 12.) The name foliated or schistose is given to a rock in 

 which the arrangement of the constituent minerals in parallel 

 planes is sufficiently marked to cause it to split in one direction 

 more readily than in any other. Not infrequently the quartzes 

 or feldspars occur in lens-shaped forms about which curve the 

 hornblende or mica folia as shown in Fig. 2, PL 12, As ex- 

 plained elsewhere, this structure may be due to original deposi- 

 tion or may be secondary. In eruptive rocks a fluidal or fluxion 

 structure is not uncommon, as shown in Fig. 2, PL 2, and is 

 due to the onward flowing of the mass while gradually cooling 

 and passing into a solid state. Eruptive magmas at the time of 

 their extrusion contain more or less moisture, which, being 

 highly heated, expands whenever sufficient force is developed 

 to overcome the pressure of the overlying mass. In this way 

 are formed innumerable cavities or bubbles, comparable to the 

 cavities caused by carbonic acid from the yeast in well-raised 

 bread. Such cavities are called vesicles, and the rocks contain- 

 ing them are vesicular (Fig. 2, PL 3). By the subsequent 

 action of percolating waters these cavities may become filled 

 with a variety of secondary minerals, among which chalcedony, 

 epidote, calcite, and various zeolites are not uncommon. Such 

 refilled cavities are called amygdules, from the Greek word 

 afxvy^aXov, an almond, in allusion to their shape, and the rocks 

 containing them are therefore described as amygdaloidal. The 

 upper part of a lava flow sometimes cools in peculiar ropy 

 forms like the slag from a smelting furnace. Such forms are 

 known as slaggy. (See Fig. 1, PL 3.) 



When a rock consists of a compact, glassy, or fine and evenly 

 crystalline ground-mass, throughout which are scattered larger 



