410 Met amor phism. 



proof from experiment, is justified in making a contrary 

 assumption. 



And the case of clays is not different. Very frequently 

 retaining all their essential features as aqueous rocks : — the 

 marks of bedding still easily seen: — the remains of plants, shells, 

 teeth, and bones, still common : — it is not rare to find some of 

 these clays converted into slates, and it is yet more usual to 

 find them alternating with calcareous or irony bands, as already 

 alluded to. These are not unusually collected round some 

 group of fossils, acting perhaps as a disposing cause for the 

 separation. The slates are yet more striking illustrations of 

 change ; and in them also occasionally shells and other fossils, 

 little altered, indicate a water origin. Water currents, acting 

 under enormous pressure, have so far altered clays that they 

 have become slates. But the veins, or cracks and crevices 

 that abound in slates, are filled with quartz in a crystalline 

 state. These are often full of holes, and within them some- 

 times iron pyrites, sometimes valuable ores of copper, make 

 their appearance. But in the deposits of ore there is still no 

 appearance that is not more readily explained by the action 

 of water than by fusion. Currents of heated water — electric 

 currents passing through the earth in definite courses — these 

 would seem to have been the agencies in producing all the 

 transformation and metamorphosis of the richest metalliferous 

 rocks of Cornwall and other countries where mining is carried 

 on in slate. 



"When we examine such rocks as granite and the large 

 group of compounds of which it is the type, we enter on more 

 doubtful ground, and must therefore tread more cautiously. 

 These have long been held as the proof of igneous action, and 

 few are prepared on first examining them to attribute any of 

 them to mere change in contact with water. But here again 

 we are bound to look to the facts of the case — esse quam viderl 

 must be our motto. 



And first there is the texture and appearance of granite 

 itself as compared with rocks we know to have been molten, 

 such as lavas, ancient or modern. Here the microscope comes 

 into use, and teaches us that a very large class of rocks, of 

 which granite is one, have, beyond all doubt, been formed 

 in contacts with water, for water in a fluid state is actually 

 mixed up with them, and is contained AvSthin the separate 

 crystals of which they are constructed. That granites, 

 syenites, and greenstones, with tho host of rocks of the same 

 or nearly the same general character, wnioh we may call por- 

 phyries for want of a better name, have been formed with 

 water, there can indeed be no longer a doubt; but it is still 

 uncertain what temperature has been needed to produce them.' 



