Handbook of Paleontology 31 



ates and breccias, the sandstones, the mudstones or 

 shales etc. To these may be added the limestones 

 secondarily of mechanical origin. 



Conglomerates are composed predominantly of rounded 

 pebbles anywhere from the size of a pea to boulders of 

 large size. In the latter case the conglomerate is known 

 as boulder conglomerate, but when the pebbles are very 

 small the rock is called a grit. These pebbles are usually 

 water-worn and of river or seashore origin, and they are 

 intermingled with a finer material known as the matrix, 

 which acts as a cement. The pebbles may consist of any 

 kind of rock, but usually they are derived from the more 

 resistant rocks and minerals. They may be all of one 

 kind of rock or mineral or there may be a variety of kinds. 

 When there is one kind of rock or mineral the conglom- 

 erate is often called bv that name, hence we have quarts 

 conglomerates, lime tone conglomerates, granite conglom- 

 erates, volcanic (lava) conglomerates, sandstone con- 

 glomerates etc. The pebbles may be abundant or few and 

 scattered, in which case the rock is known as a pebbly 

 sandstone. When conglomerates are composed of small 

 pebbles they are apt to be well stratified and may show 

 cross-bedding and other structures (see page 44) shown 

 in sandstones etc. The cementing material of conglomer- 

 ates also varies. Sometimes there is a gradation in size 

 from the pebbles into the matrix, again there may be little 

 gradation and the pebbles are sharply set off from the 

 matrix, forming what is known as pudding-stone. The 

 pebbles in a conglomerate may show a variety of colors. 

 They may be all one color or the pebbles and matrix may 

 be alike in coloring, giving one general hue to the rock. 

 The cementing material also varies to a great extent. It 

 may be a consolidated sand, either a pure quartz sand or 



