A series of rock analyses from my field work in the Tysfjord district of Northern Norway offered the opportunity for exact calculation of a numher of hornblende minerals, occurring more or less abundantly in some of the rocks. Optical determinations were carried out on the analyzed material. Afterwards a ferro-hastingsite was also analyzed chemically.
The object of the work has be en:
1. To give a contribution to the study of relations between the chemical composition and the optical properties within some parts of the complicat·ed group of common amphiboles, considering the newest concepts concerning the constitution of this group.
2. To study the composition of the amphi!boles, formed in certain metamorphic facies of higher temperature, wihich have a very wide distribution in the Caledonides of Norway.
3. To give a clue for the determination of the amphiboles, formed under these given conditions, by optical measurements alone.