446 SCIENTIFIC RECORD FOR 1883. 



ALTERED OB METAMORPHIC ROCKS. 



Brogger has also described the local changes of the Paleozoic strata 

 near Christiania, where eruptive masses of granite and of syenite have 

 caused the development of crystals of chiastolite in certain shales still 

 retaining the marks of graptolites. Other beds are changed into a 

 kind of hornstone, while in the limestones of the series the shells of 

 Orthis are found associated with well-developed garnets. Similar facts to 

 these are well known in other regions. These conditions are, however, 

 very unlike those presented in disturbed regions where a process of 

 folding and inversion having caused uncrystalline strata to pass below 

 crystalline stratified rocks, these latter have been assumed, as in the 

 Scottish Highlands, to have resulted from an alteration of still newer 

 and originally superimposed uncrystalline sediments. 



Uncrystalline rocks thus enfolded seem occasionally to have suffered 

 local changes, due apparently to the action of thermal waters coining 

 up through fractures along the folds and giving rise to crystalline min- 

 erals. E6nard has lately described an interesting example of the kind 

 in a belt of graywacke and slates referred to the Devonian, and affected 

 by a northeast and southwest plication. The change is very marked 

 along the axis of this, but shades off on either side till the sediments 

 are unaltered. Garnet, hornblende, mica, and titanite have here been 

 developed in the schists, and are associated with anthracitic matter. 

 These minerals are mixed with grains of clastic origin, and the whole 

 of the phenomena would appear to be due to infiltrating waters. There 

 is, however, a wide difference between these mixtures of clastic mate 

 rials with crystalline silicates which have been formed and deposited in 

 their midst, and the wholly crystalline feldspathic and kornblendic rocks 

 of the Eozoic ages. 



In this connection Bonney has lately studied the so-called metamor- 

 phic conglomerate of Valorsine in the Alps, and has submitted to care- 

 ful microscopic examination the layers of so-called gneiss and mica- 

 schist in this conglomerate of carboniferous age. He shows that the 

 mica and other constituent silicates of these were derived from pre- 

 existing crystalline rocks, and that the material has been subjected to 

 immense pressure, by which the quartz has been broken and the feld- 

 spar crushed. From the latter, and from interposed earthy dust, minute 

 scales of micaceous minerals have been formed by such micro-minera- 

 logical changes as are always at work in similar rocks, and chalcedonic 

 quartz has been deposited. He adds, however, that " of metamorphism, 

 in the technical sense of the word, there is no trace." 



Bonney further remarks that " a few years since it would have been 

 heresy to assert that very clear proof would be necessary before we 

 could accept a crystalline schist as the metamorphosed representative 

 of a rock of Paleozoic age. Yet at the present time many who have 

 made a special study of this branch of petrology would not hesitate to 

 go this far, and some would even declare that we do not know of any 



