24 GEOLOGICAL HISTORY. [CH. 



deposits, there would be formed a hard rock made up of 

 rounded fragments of various kinds of strata derived from 

 different sources. Such a rock is known as a Conglomerate. 

 The same kind of rock may be formed equally well by the 

 action of the sea ; an old sea-beach with the pebbles embedded 

 in a cementing matrix affords a typical example of a coarse 

 conglomerate. Plant remains are occasionally met with in 

 conglomerates, but usually in a fragmentary condition. 



From a conglomerate composed of large water- worn pebbles, 

 to a fine homogeneous sandstone there are numerous inter- 

 mediate stages. A body of water, with a velocity too small to 

 carry along pebbles of rock in suspension or to roll them along the 

 bed of the channel, is still able to transport the finer fragments 

 or grains of sand, but as a further decrease in the velocity 

 occurs, these are eventually deposited as beds of coarse or fine 

 sand. .The stretches of sand on a gradually shelving sea shore, 

 or the deposits of the same material in a river's delta, have 

 been formed by the gradual wearing away and disintegration 

 of various rocks, the detritus of which has been spread out in 

 more or less regular beds on the floor of a lake or sea. 

 Such accumulations of fine detrital material, if compacted 

 or cemented together, become typical Sandstones. 



In tracing beds of sandstone across a tract of country, it 

 is frequently found that the character of the strata gradually 

 alters ; mud or clay becomes associated with the sandy deposit, 

 until finally the sandstone is replaced by beds of dark coloured 

 shale. Similarly the sandy detritus on the ocean floor, or in 

 an inland lake, when followed further and further from the 

 source from which the materials were derived, passes by 

 degrees into argillaceous sand, and finally into sheets of dark 

 clay or mud. The hardened beds of clay or fine grained mud 

 become transformed into Shales. As a general rule, then, 

 shales are rocks which have been laid down in places further 

 from the land, or at a greater distance from the source of 

 origin of the detrital material, than sandstones or conglomerates. 

 The conglomerates, or old shingle beaches, usually occur in 

 somewhat irregular patches, marking old shore-lines or the 

 head of a river delta. Coarse sandstones, or grits, may occur 



