72 TRANSFORMATIONS OP ROCK-MATTER. 



through the mass, as a mortar to bind the incoherent par- 

 ticles into a solid rock. Every rock in the earth's crust 

 sedimentary and vulcanic is rendered more compact and 

 crystalline by this process of chemical infiltration. "Water 

 is ever permeating this outer shell, and thus solutions of 

 lime, silica, iron, and other mineral and metallic substances 

 are borne hither and thither here cementing sand and 

 gravel into grits and conglomerates, and there calcifying 

 a sandstone; here silicifying and hardening some earthy 

 limestone, and there converting volcanic dust and ashes into 

 stony tufas and amygdaloids. Besides simply cementing 

 and hardening the masses through which they percolate, 

 these solutions give rise to new chemical actions, by which 

 certain rocks are rendered more crystalline, and altogether 

 changed in their texture. A solution of silica, for example, 

 may permeate a porous sandstone, and by cementing to- 

 gether its particles render it merely harder and more com- 

 pact ; while a similar solution, in passing through an earthy 

 chalk, might form a chemical union with the mass, and 

 convert it into a tough and flinty chertstone. Those ac- 

 quainted with the changes that can be artificially induced 

 by chemical action, can have little difficulty in conceiving 

 how vast and complex the metamorphoses that may be 

 wrought within the rocky crust by the same agency, and all 

 the more that heat and moisture are there ever present to 

 facilitate its operations. And even where the changes are 

 slow and gradual so slow as to be almost imperceptible 

 we may rest assured they are still going forward, and only 

 require time for their perfect development. The recent 

 rocks must necessarily have suffered less metamorphism 

 than the ancient, but still the time and fitting conditions 

 will come when the loose sand will be converted into 

 sandstone, and the sandstone, by further change, into 

 quartzite; the peat be changed into lignite, the lignite 



