STAGES IN METAMORPHISM. 359 



seated rocks, by which the carbonic acid was retained in them and 

 the particles of the rock entirely rearranged. Similarly the change of 

 loose sand or argillaceous sandstone to solid sandstone or quartzite, 

 or jasper, or rhyolite, or granite, is, in the first place, a case of cementa- 

 tion of the grains through heat, followed by gradual solution of the 

 rock in the water which it contained, and more or less perfect crystal- 

 lisation, by which in many cases twin crystals of highly complex 

 structure have been formed by the enlargement and blending of 

 crystals which were at first microscopic. In the consolidation of clay 

 slate, the particles are not merely pressed together, but more or less 

 confluent at the edges, from crystallisation, 1 and in basalt and basaltic 

 dykes we see a more perfect development of molecular metamorphism. 



Stages in Chemical Metamorphism. The most extreme change 

 induced by heat, and the chemical actions which heat and water 

 acting under pressure set up, is an alteration in the nature of a 

 rock. Such a case, in its simplest form, may be typified by the gene- 

 ration of garnet in the vicinity of dykes and large igneous masses, 

 or in the artificial combinations of the furnace. Near the dyke of 

 Plasnewydd, in Anglesea, Professor Henslow collected in the altered 

 and jasperised shales grey garnets ; in the rock of the mountain called 

 the Gable, near the granites of Wast Water, are multitudes of beauti- 

 ful red garnets. We are thus led by easy analogy to view the in- 

 numerable garnets in the mica slate of the Highlands as generated in 

 those foliated rocks by chemical combinations which originated under 

 the same influence of heat, as that by which the limestone which lies 

 in them has become crystalline, and by which the schists have acquired 

 their granitoid aspect. 



Ratio of the Metamorphism of Strata. On considering the 

 series of strata in relation to the degree of their metamorphism, it 

 is impossible not to perceive that in most countries metamorphism 

 increases continually with the age of the rock. It is impossible at 

 present always to point out exactly the amount of changes which 

 have been produced on the primary strata by the general and con- 

 tinued communication of heat from below; gneiss, for example, is 

 in some cases almost identical with granite, in other cases approxi- 

 mates to sandstone. Yet when we consider the bedded and 

 laminated character of this rock, and observe that its constituent 

 minerals, even when united into a dense rock, are not crystallised 

 with regular external forms, successively modifying one another in a 

 certain order, we understand that the rock has been solidified by 

 a species of crystalline growth at the edges of the constituent sub- 

 stances, which, carried to extreme, under the requisite pressures, 

 would have reconverted the whole into granite. 



Similar remarks apply to mica schist, w^hich, on the one hand, 

 varies to gneiss, and on the other to clay slate ; and it is observable 

 that the fusible mineral garnet, which is generated at moderate heats 

 in rocks in contact with basalt, is very generally intermixed with the 

 laminae of gneiss and mica slate. 



1 Sorby, Address Geol. Soc., 1880, p. 47. 



