ANNIVERSAET ADDRESS OF THE PEESIDEFT. 8 1 



Metamorjphic Hocks. 



The above-named facts naturally lead me to tlie question of me- 

 tamorphism. If we had to deal with such extremes as shale and 

 schists, we could not hesitate as to the use of the term metamorphic ; 

 but, as so often happens, one extreme condition passes so gradually 

 into another that it is difficult or impossible to draw a boundary 

 line. I must say that I feel quite unable to offer such a definition 

 of metamorphism as would agree with the microscopical structure 

 of the rocks, and satisfy the requirements of field -geologists. If 

 its original constituents have been entirely recrystallized into new 

 minerals, especially into quartz and anhydrous silicates, it seems 

 quite reasonable to call a rock metamorphic ; but such a definition 

 would include many rocks which are usually called slates, simply 

 because no such complete recrystallization can be detected by the 

 naked eye. On the contrary, many schists, which are always called 

 metamorphic, do, as I believe, sometimes contain material which 

 has been little, if at all, changed ; so that the prevalent definition of 

 metamorphism seems to depend more on general external appear- 

 ance than on internal structure. However, instead of occupying 

 time in discussing names, it will be far better to consider a few 

 points which appear to deserve special attention. The entire ques- 

 tion of metamorphism is far too wide to be dealt with on such an 

 occasion as the present. 



Artificial Metamorphism. 



I much regret that I am unable to refer to more than one 

 illustration of metamorphism produced artificially. This is a spe- 

 cimen of slate kept long at a high temperature by our late, lamented 

 friend David Forbes. Unfortunately I do not know the exact 

 nature of the rock before it was changed ; but it is quite clear that 

 the heat has actually fused some portions and converted them into 

 what may be called slag, without having so far melted the whole 

 as to obliterate the original stratification. In the fused portion 

 needle-shaped crystals have been developed, which have the general 

 characters of the pyroxene found in some artificial slags, as well as 

 others which correspond with felspar. It is therefore quite unlike 

 any true natural metamorphic schist, but still is of interest as 

 showing what characters are due to crystallization in situ ; and the 

 newly-formed minerals closely correspond with those found in some 

 rocks metamorphosed naturally. The main difference seems to be 

 that the natural rocks were not actually fused like a slag, and, 

 being under great pressure, a certain amount of liquid water was 

 present, which greatly facilitated the transference of the consti- 

 tuents from one part of the rock to another, and the development 

 of certain minerals like quartz, mica, hornblende, and calcite. 



Local Contact Metamorphism. 



As an illustration of a few special facts, I select the sandstone 

 and shale of Carboniferous age in contact with the igneous rock of 



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