78 ELEMENTARY GEOLOGY 
ates, etc. There may also be volcanic conglomerates, 
composed of the larger fragments of volcanic pumice 
and ash. 
Sandy Rocks. As the wind blows over a desert 
country (Fig. 59), or across a beach, it may pick up 
particles of sand and pile them into hills (Fig. 62), or 
Fic. 30. 
A coquina rock. 
spread them over the land in layers. The current of 
a river, or the waves of lake or sea may gather sand 
into beds (Fig. 31). When consolidated, these sand 
beds form sandstones (Fig. 32). 
In texture these may be coarse, grading into a con- 
glomerate (Fig. 26), or very fine in grain, almost like 
a clay (as in the bluestone which is used for flagging). 
They are usually composed of grains of quartz, the 
