Sujet No. 4: L'origine et importance des sediments precambriens.

1. G. A. J. COLE, Illustrations of the formation of composite gneisses and amphibolites in northwest Ireland (page 311).

2. J. J. SEDERHOLM, Different types of Pre-Cambrian unconformities (page 313).

3. J. J. SEDERHOLM, On regional granitization (or anatexis) (page 319). Discussion.

4. W. S. BAYLEY, The Pre-Cambrian sedimentary rocks in the highlands of New Jersey (page 325).

5. C. K. LEITH, Relations of the plane of unconformity at the base of the Cambrian to terrestrial deposition in late Pre-Cambrian time (page 335). Discussion.

ILLUSTRATIONS OF THE FORMATION OF COMPOSITE GNEISSES AND AMPHIBOLITES IN NORTH

WEST IRELAND.

BY

GRENVILLE A. J. COLE,

Director of the Geological Survey of Ireland, Dublin.

The Dalradian series, so appropriately named by Sir A. GEIKIE, is represented in northwest Ireland by well bedded quartzites, shales (now mica-schists), and limestones, penetrated by basic igneous rocks, which are now in the condition of aphanites and diorites. Masses of granite have greatly modified this series, producing contact effects and composite gneisses on a regional scale. One of these granites commonly agrees in the general trend of its masses with the axes of Caledonian folding, while a later granite cuts across it. Both, however, may be of Pre-Cambrian age.

The banded gneisses of the area, which in hand specimens resemble much of the so-called "fundamental gneiss" of other countries, are clearly traceable into the sedimentary rocks, from which they have been derived by intimate injection of granitic material parallel with the layers of the stratification. In many places, as at Lough Derg in southern Donegal, the only relics of the absorbed Dalradian series are rounded or ellipsoidal masses of amphibolites (quartz-felspar-pyroxene-amphibole rock). These, no doubt, often represent undigested blocks of aphanite, but may in part be derived from limestone, as was discussed in a paper in the Transactions of the Royal Irish Academy in 1900. In the long ridge of the Ox mountains, the amphibolites are often drawn out by the flow of the granite into sheets, and the contortion of these sheets during the general softening and intermingling has produced a very striking gneissose structure near Ballintogher and Drumahair in County Leitrim. This has been ascribed by certain authors to dynamo-metamorphic action; but the stages that can be traced from the granite with blocks of amphibolite to the banded gneiss afford convincing evidence. of the composite origin of the gneiss under the attack of the advancing granite magma.

Occasionally, as at Doocharry Bridge in County Donegal, only isolated sheets of mica-schist remain in a granitic mass; but these retain their parallelism with the strike of the sedimentary series, and are the last surviving relics of the Dalradian cover that has been, for the most part, eaten into and absorbed.

A striking example of the effect on bedded quartzite of nearness to a granitic cauldron was observed at Minaun Cliffs, Achill Island, County

Mayo, during the visit of the Geologists' Association in 1912. The strata remain horizontally bedded in the great cliff section, except in their lower part, where veins of a coarse red granite penetrate them near the present shore line. The quartzites have here undergone intense crumpling and overfolding, such as one meets with on a large scale in mountain ranges, and this contorted flow seems to be entirely due to the yielding that has taken place in the region of heating on the upper surface of the granite cauldron.

The oldest rocks known in northwest Ireland are the Dalradian sedimentary series, and here, as in so many other countries, there is no evidence of the existence of anything like a “fundamental gneiss.”

DIFFERENT TYPES OF PRE-CAMBRIAN

UNCONFORMITIES.

BY

J. J. SEDERHOLM,

Director of the Geological Survey of Finland.

Where Pre-Cambrian sedimentary rocks have beeen found in great thicknesses and studied in some detail, they have usually been found to include several different series separated by unconformities. I shall here endeavour to show, by examples from the region which I have studied myself, what different types are represented by these unconformities.

THE SUB-CAMBRIAN UNCONFORMITY.

If we regard the Olenellus-zone as the bottom layer of the Cambrian, we find, both in Esthonia and in Scandinavia, sedimentary rocks underlying it in apparent or real conformity, viz., the Blue Clay, with its associated sandstones, and the Sparagmite of the Scandinavian mountains. While the Sparagmite, a coarse feldspathic sandstone of the same type as the Torridonian of Scotland, again reposes directly, and in apparent conformity, on the Jotnian Dala sandstone of Sweden, we find in the east a more marked unconformity between the Eo-Cambrian clay and its basement rocks. In St. Petersburg, borings have shown that the Blue Clay is in immediate contact with an underlying red gneissoid granite. Hence the Jotnian sandstones, in a zone which in all probability extended over a considerable area in southern Finland and in the vicinity of St. Petersburg, must have been removed by erosion before the deposition of the Cambrian.

The peculiar rapakivi granite of the island of Aland, between the Baltic and the Gulf of Bothnia, contains fissure veins in which have been found fossils belonging to the Olenellus fauna, from which it appears that the granite was exposed at the time of the deposition of the Olenellus sandstone. In Sweden, the rapakivi granite is covered by Jotnian sandstone, and the same has probably been the case also in western Finland, although it has been removed later by erosion.

Thus we find several supports for the conclusion, that there was a prolonged period of erosion before the beginning of the Cambrian, during which the Archæan and Jatulian were levelled, the youngest Pre-Cambrian granites uncovered, and the Jotnian sandstones deposited and in great part again worn away.

The apparent conformity between the Cambrian, the Sparagmite and the Jotnian sandstones in Scandinavia may, therefore, possibly be deceptive.

In any case, the Jotnian of eastern Fenno-Scandia was deposited long before the beginning of the Cambrian period.

Among the sedimentary rocks of northern Norway, in the formations which have been called Raipas and Gaisa, there may be deposits which fill up the gap between the Jotnian and the Cambrian, and between the former and the Jatulian. Also in southern Fenno-Scandia there are certain sedimentary rocks which lie horizontal like the typical Jotnian, but which are certainly older than the main part of the Jotnian. Thus, on the island of Hogland there are conglomerates and sandstones at the base of a great sheet of quartz-porphyry which is genetically connected with the rapakivi granite. The time of deposition of these sediments is separated from that of the Jotnian by the time required for the eruption and erosion of the quartz porphyry. The sandstones of Almesåkra in Sweden, in part slightly metamorphic, may also belong to the same Lower Jotnian rocks.

In all these cases we have to do with rocks which are entirely similar to younger sediments and have suffered very little disturbance in the region east of the Scandinavian mountain ridge. The unconformity between them and the underlying rocks is of the same character as, for example, that between the Vosgian sandstone of Baden and the underlying granite.

THE SUB-JATULIAN UNCONFORMITY.

Passing now to the underlying older formations, the quartzitic sandstones, conglomerates, argillites, dolomites, etc., belonging to the Jatulian system, we again find these well separated by unconformities from the overand underlying rocks.

1

The Jotnian rocks have never been found in direct contact with the Jatulian, but in Olonetz, to the northwest of Lake Onega, both systems are found near each other, the older always compressed into gentle folds and the younger, in the continuation of the strike of the former, lying nearly horizontally. The basic igneous rocks intercalated between their beds are, in the younger formation, unmetamorphosed, in the older always more or less completely uralitized. Moreover, the folded quartzitic sandstones and the associated basic rocks are penetrated by the unmetamorphosed rapakivi granites, whose eruption, as already stated, happened in Lower Jotnian time. These facts show that the folding of the Jatulian rocks, which took place over the whole area of eastern Fenno-Scandia and probably also over the western parts of it (HÖGBOM and other Swedish geologists ascribe a Jatulian age also to the folded quartzitic sandstones, etc., of Dalsland, on the western shore of Lake Wenern in Sweden), happened in pre-Jotnian time. There seems to be now a general agreement among Scandinavian and Finnish geologists as to this point.

The Jatulian rocks, which comprise thicknesses of at least 3,000-5,000

1 W. RAMSAY separates the uppermost parts of the Jatulian, including also the coalbearing rocks of Schunga in Russian Carelia and Suojärvi in Finland, from the remainder, calling them Onegian.