300 NEW ZEALAND NATURE-STUDY BOOK 
A number of specimens taken at random from a gravel 
pit, a sea-beach, ora river bed will usually include different 
kinds of rock, 
Notice the smooth rounded surface of each stone. Educe 
that pieces of rock break off and tumble into the sea or 
river beds—that these are rolled over and ground together 
by the action of the waves or river currents, that the 
angular projections are first removed, and that the grinding 
process continues until the stone is rounded and smooth. 
Examine a few of the separate stones, and notice that 
they are not all composed of the same material. 
Educe where Gravel is most likely to be found. Why 
on sea-beach or in river-bed ? Why are the stones in the 
lower courses of rivers more rounded and smooth than 
those near the source ? 
CONGLOMERATE. 
Conglomerate is a mass of consolidated Gravel or 
Shingle, the pebbles being rounded or waterworn, and 
set in a paste or cementing matrix which binds them 
together into a hard mass. The pebbles may consist 
of any kind of hard rock or mineral, which is able to 
resist the grinding process to which it is exposed. A 
special name may be given according to the nature of 
the pebbles—when these consist chiefly of Quartz, the rock 
-is calleda Quartz-conglomerate; when they are Limestone, it 
is a Limestone-conglomerate, and so on. The matrix 
binding the pebbles also varies in composition—sometimes 
it is merely sand, and the rock seems to have been con- 
solidated simply by pressure. In other cases it consists of 
cement which has set so hard that a blow produces a clean 
fracture through both matrix and pebbles. The cementing 
material may be Silicious, Calcareous, Clayey or Ferru- 
ginous (abounding in Iron). 
