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44 ' Bulletin de la Uommission géologique de Finlande N:o 128. It seems, therefore, that the hypothesis of a deep erosion in early Archaean periods, before the mise-en-place of the Svecofennidic granites, must be considered, although there is no direct evidenoe of such an event registered in the rocks. We know of no widely distributed conglomerates containing pebbles of deep-seated intrusive rocks, as might be expected, and we know of no great unconformities. Never the less, the existence of the quartzite formation is in itself an evidence of great things that have happened in those remote ages. Perhaps it represents occasional remainsfrom times so remote that almost all geological record has been lost. We must remember the fact that we do not know with certainty any granites older than the leptite formation, to whose age-group the Tiirismaa quartzite as well as other remains of old Archaean quartzites would seem to belong. And yet there is good reason to maintain that the volcanics of the leptite formation have been erupted through a granitic crust. Great gaps must exist in the series of the oldest strata of our globe - as wdi as in the geological information about these things! Quite another way to solve the riddle of the Tiirismaa quartzite would be to suppose it to be Karelidic in age, in which case the surrounding granites would be post-Karelidic. Such a hypothesis, however, is contradicted by the cardinal multa of J3mish geology to such an extent that it can hardly be taken into consideration. In their mineralogical development the supracrustal metamorphic rocks surrounding the quartzite represent two different kinds of mineral facies. The conglomerates, epidote-quartzites, and quartzitic gneisses in the east and northwest are characterized by epidote and palecoloured iron-poor amphiboles, and comparatively much titanite, and chlorite. All the silicates, i. e. epidote, hornblende, and biotite, are poor in iron, a fact that cannot be merely accounted for by the absence of iron, as most of the rocks contain appreciable quantities of oxidic iron ore. The greenschist at the southwestern side of the quartzite contains deep green hornblende in process of alteration into epidote and chlorite. In both rocks the epidote-chlorite association is later than the hornblende with which anorthitic plagioclase, even bytownite, is found on the east side, while albite is the chief component of the greenschiat on the soutwest side. These circumstances can only be interpreted by assuming two different processes of metamorphism, an earlier amphibolite metamorphism and a later retrogressive greenschist metamorphism. If the former is connected with the intrusion of granites and the migmatization, the latter must be due to a temperature that had fallen to a certain degree. It seems as if the quartzite which had succeeded in resisting the attack of the granite
Suomen Geologinen Seura. N:o 12. Qeologiska Sällskapet i Finland. 45 magma had absorbed and conducted heat alao from the surrounding rocks. Has this been due to the better themal conductivity of the quartzite as compared with the surroundic migmatites? This would seem quite probable if the quartzite, as a huge mass, was continuous up to the land-surface of that time. This hypothesis would gain in probability if it could be proven that big masses of quartzite in folded complexes are commonly associated with rock of some low temperature facies in their nearest surroundmgs. This possibility might be worth co~midering. The Mineralogical and Geological Institute of the University, Helsinki, January 1938. .
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