IGNEOUS KOCKS 47 



out on the surface of the earth in a molten condition. Such 

 are characterized by less perfect crystallization and a more 

 slaggy and vesicular structure than the deep-seated rocks. A 

 columnar jointing, due to cooling, is by no means unconunon, 

 particularly among basaltic lavas, although it is by no means 

 confined to them. 



But a comparatively small proportion of the rocks composing 

 the superficial portions of the earth's crust — the portions with 

 which we are more or less familiar — are eruptive. They are 

 rather what are known as secondary rocks; that is to say, they 

 are rocks made over from these so-called primary rocks, which 

 we have been just discussing, by processes which will be described 

 later. 



Any rock mass, be it eruptive or otherwise, lying exposed at 

 or near the surface of the ground finds itself subjected to a 

 multitude of disintegrating and decomposing agencies, which 

 are described more in detail under the head of rock weathering. 

 Leached and decomposed by meteoric waters, disintegrated by 

 heat and frost, or the mechanical action of waves and currents, 

 the rock masses slowly succumb, their materials being in part 

 removed in solution, or as debris mechanically transported by 

 every wind, rain, or running stream, down the slopes into 

 the valleys, and from the valleys into the seas. This debris, in 

 various stages of coarseness and fineness, to which we give 

 the name of bowlders, gravel, sand, or silt, undergoes by these 

 transporting agencies a system of assorting more or less com- 

 plete, and is carried to distances dependent upon its weight and 

 the force of the transporting agent. It requires no geological 

 or other special training to enable one to understand that the 

 force being the same, the finer and lighter materials will be 

 carried farthest, and that all must be deposited when the force 

 shall be expended. Consider, then, -for purpose of illustration, 

 a stream flowing from a mountainous region and emptying 

 itself into the sea. Materials falling by gravity from the moun- 

 tain slopes, or washed by spasmodic rains into the stream, are 

 transported certain distances, according to the strength of the 

 current. For present purposes, it is sufficient to consider only 

 those portions which are transported quite to the mouth of the 

 stream and dumped into the sea. But as the water leaves its 

 narrow channel there is an almost instant diminution of the force 

 of its current, and consequent carrying power. As a result, it 



