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The rocks of western Connecticut can be divided into three general categories: (1) the Precambrian complexes (Housatonic and Berkshire Highlands Gneiss complexes), (2) the metasedimentary rocks west of a major structural break known as Cameron’s line, and (3) the lower Paleozoic rocks east of Cameron’s line. The core of the Waterbury dome (Waterbury Formation), though east of Cameron’s line, is older than the mantling lower Paleozoic rocks.

The Precambrian rocks are interlayered granitic to dioritic gneisses and amphibolites and mafic gneisses with subordinate terrigenous metasediments and are separated readily from the lower Paleozoic sequence.

The Waramaug Formation, probably the oldest part of the Paleozoic sequence, is a belt of interlayered and intergradational mica-plagioclase-quartz gneiss and sillimanite-garnet-mica-plagioclase-quartz gneiss, representing original graywackes and shales. This belt wraps around the Precambrian highlands and lies to the west of Cameron’s line between the Precambrian rocks and the eastern Paleozoic rocks.

The Waterbury Formation, exposed in the core of the Waterbury dome, is an assemblage of metasedimentary and metavolcanic rocks of unknown thickness representing sedimentary mixtures of quartz and plagioclase sands, clay minerals, volcanogenic material, coarser clastic material, and possibly flows. These rocks are most similar in lithology to the belt of miogeosynclinal metasedimentary rocks west of Cameron’s line and may be their eugeosynclinal equivalent. The Rb-Sr whole-rock data on these rocks give an isochron age of 465 ± 50 m.y. (Clark and Kulp, 1968).

The rocks in the belt east of Cameron’s line constitute a continuous sequence of predominantly eugeosynclinal sediments that is divided into five mappable units. The lowest unit (Hartland Unit I) is predominantly fine-grained gray mica-plagioclase-quartz granulite and granulitic gneiss with subordinate schist layers. The second unit (Hitchcock Lake) is a banded and streaked rock composed of alternating thick and thin layers of a fine-grained plagioclase-quartz granulite and coarsely streaked micaceous plagioclase-quartz gneiss. The third unit (Hartland Unit II and The Straits Schist) is a lustrous muscovite-plagioclase-quartz schist with large porphyroblasts of garnet and kyanite or staurolite. The fourth unit (Hartland Unit III) is a thinly interlayered, heterogeneous assemblage of fine-grained muscovite-biotite-plagioclase-quartz granulite and muscovite-plagioclase-biotite-quartz schist; many of these rocks are unusually graphitic. The fifth unit (Hartland Unit IV) is also as assemblage of thick and thin layers of granulites and schists with minor garnet-staurolite-bearing units.

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