20 PROCEEDINGS OF WASHINGTON MEETING. 



A brown mica referred to biotite has been formed abundantly in many of the 

 rocks studied : sometimes directly from augite, more often from the decomposition 

 products of that mineral and the feldspars. Besides the flocculent clusters of scales 

 occupying the place of vanished pyroxene, there are often minute flakes of biotite 

 scattered through the regenerated feldspars alluded to above. In spots where more 

 lime was present, as, for instance, within the vesicles of the lavas and in cer. 

 tain little veins which must have been occupied in part by calcite, green hornblende 

 occurs instead of biotite ; and some little veins, more calcareous than the others, are 

 converted instead into a granular monoclinic pyroxene. This pyroxene must, from 

 chemical considerations, be one rich in alumina as well as lime — an omphacite rather 

 than a diopside. The distribution of these various minerals in the metamorphosed 

 volcanics is a good illustration of the way in which the products formed at any 

 point depend on the chemical composition of the mass in the immediate neighbor- 

 hood of that point. Prior to metamorphism certain substances were uniformly dis- 

 tributed through the rock, while others, owing in great measure to weathering 

 action, were concentrated in particular spots; from this results in part the wide 

 variety of secondary minerals frequently met with. 



The titaniferous minerals afford another instructive example. Titanic acid in 

 some form seems to have been pretty uniformly distributed through many of these 

 old volcanic rocks. In the metamorphosed products it is for the most part taken 

 'up by the mica (typical biotite containing nearly "i per cent, of titanic acid) ; but 

 where there has been sufficient lime to form hornblende or omphacite in place of 

 biotite, the titanic acid appears as sphene ; where iron oxides were present in some 

 abundance we find ilmenite ; and again, in some of the rhyolitic ashes very poor 

 in lime and iron, simple rutile occurs. Such facts certainly point to the conclusion 

 that in the processes of thermometamorphism there is very little interchange of 

 substance except between closely adjacent points. 



Of some significance in this connection is the constant preservation of the former 

 structures of the rocks, despite extreme metamorphism of their material. The 

 ovoid vesicles of the andesites, filled previous to the granitic intrusion by the ordi- 

 nary weathering products, are still perfectly distinct even in the most highly meta- 

 morphosed examples. The flow-structures of the lavas and the lamination of the 

 ashes, whenever they were distinctly pronounced, have been well preserved, being 

 often emphasized by a certain foliation due to the parallelism of biotite flakes. 

 and then indistinguishable from typical micaschists or microgneisses. In places 

 where the rocks have been cleaved before metamorphosis, this foliation follows the 

 cleavage. The macrospherulites in the rhyolites have at an early date undergone 

 changes common in the older acid lavas, giving rise to a segregation of different 

 materials in alternating concentric shells, and tins structure is beautifully retained 

 in the metamorphosed examples, the several distinct shells giving rise to different 

 secondary products, and the concentric partings being defined by special minerals 

 due to " agents mineralisateurs." A calcareous breccia overlying these acid lavas 

 contains angular fragments of rhyolite, and these, even close to the granite, retain 

 their micro-spherulitic and other structures, besides a system of minute perlitic 

 cracks now occupied by little veins of pyroxene which clearly represent calcareous 

 infiltrations from the matrix of the breccia. Such striking instances of the preser- 

 vation of minute structures negative the idea of any considerable interchange of 

 material between different parts of the rocks affected, and by implication suggest 



