658 THE POPULAR SCIENCE MONTHLY. 



differs from any Palaeozoic sandstone or quartzite tliat I have yet 

 seen. Among grains of quartz and feldspar are scattered numer- 

 ous flakes of mica, brown or wliite. The form of these is so regu- 

 lar that I conclude they have been developed, or at least completed, 

 in situ. Moreover, the quartz and the feldspar no longer retain 

 the distinctly fragmental character usual in a Palaeozoic grit, but 

 appear to have received secondary enlargement. A rock of frag- 

 mental origin to some extent has simulated or reverted to a truly 

 crystalline structure. In regard to the larger fragments we can 

 affirm that they were once granitoid rock, but in them also we 

 note incipient changes, such as the development of quartz and 

 mica from feldspar (without any indication of pressure), and there 

 is reason to think that these changes were anterior to the for- 

 mation of the pebbles. To sum up the evidence : In the oldest 

 gneissoid rocks we find structures different from those of granite, 

 but bearing some resemblance to, though on a larger scale than, the 

 structures of vein-granites or the surfaces of larger masses when 

 intrusive in sedimentary deposits. We find that pressure alone 

 does not produce structures like these in crystalline rocks, and 

 that when it gives rise to mineral banding this is only on a com- 

 paratively minute scale. We find that pressures acting iipon 

 ordinary sediments in Palaeozoic or later times do not produce 

 more than colorable imitations of crystalline schists. We find 

 that when they act upon the latter the result differs, and is gen- 

 erally distinguishable frojn stratification-foliation. We see that 

 elevation of temperature obviously facilitates changes and pro- 

 motes coarseness of structure. We see also that the rocks in a 

 crystalline series which appear to occupy the highest position 

 seem to be the least metamorphosed, and present the strongest re- 

 semblance to stratified rocks. Lastly, we see that mineral change 

 appears to have taken place more readily in the later Archaean 

 times than it ever did afterward. It seems, then, a legitimate in- 

 duction that in Archaean times conditions favorable to mineral 

 change and molecular movement — in short, to metamorphism — 

 were general, which in later ages have become rare and local, so 

 that, as a rule, these gneisses and schists represent the foundation- 

 stones of the earth's crust. On the other side, what evidence can 

 be offered ? In the first place, any number of vague or rash asser- 

 tions. So many of these have already come to an untimely end, 

 and I have spent so much time and money in attending their exe- 

 cutions, that I do not mean to trouble about another till its 

 advocates express themselves willing to let the question stand or 

 fall on that issue. To a geologist (especially one belonging to the 

 school of Lyell) it is equally difficult to conceive that there should 

 be a broad distinction between the metamorphic rocks of Archaean 

 and post- Archaean age respectively, as that the pre-Tertiary vol- 



