UNSTRATIFIED OR IGNEOUS ROCKS. 223 



Amygdaloid. The upper part of a lava-stream is ve- 

 sicular, or full of air-bubbles. If such a stream be cov- 

 ered by another stream, percolating waters, charged with 

 silica and carbonate of lime gathered from the lava, will 

 fill up the empty spaces with these materials. If the rock 

 be broken or weathered, these amygdules fall out. They 

 look somewhat like pebbles, and the rock (Fig. 133) might 

 be mistaken for conglomerate, but is formed in an entirely 



FIG. 133. Amygdaloid. 



different way. The filling of the cavities takes place 

 slowly, layer within layer, and the layers are often of dif- 

 ferent colors. It is in this way that are formed the most 

 exquisite agate and carnelian nodules. 



Tufas. When volcanic materials disintegrate, and are 

 then moved and deposited in water, they form tufas. 

 Sometimes the fragments may be larger and the mass may 

 simulate volcanic breccia. It is, however, an aqueous 

 breccia of volcanic rock. Such are sometimes called vol- 

 canic agglomerates. 



