421 



We have now to consider the effect of dynamic inetamorphism on 

 the chemical and mineralogical composition of the rock. In many 

 cases the bulk-analysis of the metamorphosed rock is practically the 

 same as that of the original rock. This, however, is not so in all 

 cases. Nothing is more common than to find veins of quartz, quartz 

 and albite, quartz and orthoclase (pegmatite), and other mineral aggre- 

 gates in areas of great dynamic metamorphism. These veins evidently 

 prove that a considerable migration of chemical constituents may take 

 place, and if so an alteration in the composition of large masses of 

 rock is a necessary consequence. BROGGER has shown that quartz veins 

 increase in abundance as the faults which bound the Christiania Fjord 

 in many places are approached, and that the gneissose rocks pass into 

 a compact and highly siliceous rock resembling hornstone. Here we 

 clearly see that " silicification " is connected with a somewhat special 

 type of dynamic metamorphism. 



Somewhat different from the question of chemical change is the 

 question of mineralogical change. Old minerals are frequently destroyed, 

 and new minerals are formed. Hornblende takes the place of 

 pyroxene and olivine; epidote and zoisite are developed; basic 

 felspars of the lime- soda group give rise to albite and other minerals. 

 Of all the secondary minerals developed in connection with dynamic 

 metamorphism, the micas, both black and white, are, perhaps, the most 

 important. They are especially characteristic of those planes along 

 which the movement has been greatest. The mica-schists represent 

 the extreme of dynamic metamorphism, and they may be produced 

 both from sedimentary and igneous rocks. The white micas are 

 especially developed at the expense of the felspar; the dark micas 

 at the expense of the ferro-magnesian minerals and of garnet. 



It is very often difficult to distinguish between the original and 

 secondary minerals. The latter are usually fresher, and do not, as a 

 rule, give evidence of having suffered from mechanical action. They 

 have the aspect of minerals which have been developed in sitd. In 

 applying this test, however, great caution is requisite, because move- 

 ments have occurred again and again in the same area. The last-formed 

 minerals in metamorphic rocks (e.g., pyrite in slate and halleflinta-like 

 rocks; garnet in mica-schist) are often perfect in form. In this respect 

 we note a marked difference between igneous and metamorphic rocks. 



In conclusion we have to refer to the effect of dynamic metamorphism 

 on the mutual relations of rock masses. Consider first of all the case of 

 igneous dykes and sheets in sedimentary strata. Under normal conditions 

 of intrusion the boundary is perfectly sharp. The igneous rock near the 

 junction is, in general, finer in grain, but shows no schistosity. Sometimes 

 fluxion structure is developed, but this, in general, may easily be dis- 

 tinguished from secondary schistosity. Such dykes and sheets in disturbed 

 districts often possess very different characters. The central portion may 

 be as massive as when the rock was first consolidated. The marginal 

 portions often possess a secondary schistosity parallel with the cleavage in 



