656 THE POPULAR SCIENCE MONTHLY. 



vioiTsly crystallized can produce a foliation ; but when it has acted 

 in Palaeozoic or later times, the resulting structures can be iden- 

 tified, and these, as a rule, are distinguishable from those of the 

 most ancient foliated rocks, while at present we have found no 

 I^roof that pressure alone can produce any conspicuous mineral 

 banding, I am aware that this statement will be disputed, but I 

 venture to state, as one excuse for my temerity, that probably few 

 persons in Great Britain have seen more of crystalline rocks, both 

 in the field and with the microscope, than myself. So, while I do 

 not deny the possibility of a well-banded rock being due to press- 

 ure alone, I unhesitatingly affirm that this at present is a mere 

 hypothesis — a hypothesis, moreover, which is attended by some 

 serious difficulties. For, if we concede that, in the case of many 

 rocks originally granular, dynamic metamorphism has produced 

 a mineral banding, this is only on a very small scale ; the layers 

 are but a small fraction of an inch thick. No one could for a mo- 

 ment confuse a sheared granite from the Highlands with a Lau- 

 rentian gneiss from Canada or with an uninjured Hebridean 

 gneiss. For the former to attain to the condition of the latter, 

 the mass must have been brought to a condition which admitted 

 of great freedom of motion among the particles — almost as much, 

 in short, as among those of a molten rock. Clearly the dynamic 

 metamorphism of Palaeozoic or later ages appears to require some 

 supplementary agency. Can we obtain any clew to it ? I have 

 already mentioned the effect produced by the intrusion of large 

 masses of igneous rocks upon other rocks. These may be either 

 igneous rocks already solidified or sedimentary rocks. The former 

 may be passed over, as they will not materially help us. In regard 

 to the latter, the results of contact-metamorphism, as it is called, 

 as we might expect, are very various. Speaking only of the more 

 extreme, we find that sandstones are converted into quartzites ; 

 limestones become coarsely crystalline, all traces of organisms 

 disappearing and crystalline silicates being formed. In clayey 

 rocks all signs of the original sediments disappear, crystalline 

 silicates are formed, such as mica (especially brown) garnet, anda- 

 lusite, and sometimes tourmaline ; feldspar, however, is very rare. 

 Fair-sized grains of quartz appear, either by enlargement of origi- 

 nal granules or by independent crystallization of residual silica. 

 It is, further, important to notice that, as we approach the surface 

 of the intrusive mass — that is, as we enter upon the region where 

 the highest temperature has been longest maintained — the sec- 

 ondary minerals attain a larger size and are more free from adven- 

 titious substances — that is, they have not been obliged, as they 

 formed, to incorporate pre-existing constituents. The rock, indeed, 

 has not been melted down, but it has attained a condition where a 

 rather free molecular movement became possible, and a new min- 



