The Highland Boundary Fault is considered to extend from Stonehaven in Scotland to the major Leek fault of Clare Island in western Ireland. This conspicuous fault of the island has however been erroneously correlated with the Leannan fault of Donegal, a probable branch of the Great Glen Fault. South of the Leek fault Silurian sediments rest unconformably upon a metamorphic basement. This consists of amphi-bolite facies metasediments intruded by basic and ultrabasic rocks that have also undergone amphibolite facies metamorphism. These high grade rocks of uncertain age are considered to be part of a horst which was uplifted within the Moine Dalradian basin of western Ireland during Cambrian times. The horst was probably a southern source of detritus for the Upper Dalradian turbidites now on both sides of the Leek fault. The large uplift to the south which formed this horst conforms with and expands previous ideas on the early history of the Highland Boundary Fault. The age of the Dalradian metamorphism in western Ireland is reconsidered, and it is concluded that the case for a pre- Nitidus Zone age is unsatisfactory and that a mid-Ordovician age is possible.

The Silurian succession of Clare Island, consisting of at least 1700 m of red siltstones, sandstones and conglomerates resting unconformably on Upper Dalradian and older basement, records a cycle of marine transgression and regression within marginal marine and fluviatile sedimentary environments along the northern margin of the Silurian basin of Ireland. Clastic and pyroclastic sediments of probable Wenlock age were derived from a northern land mass formed of high grade metamorphic rocks cut by acid volcanic centres. It is argued that the source area has been removed by large scale strike-slip fault movement along a fracture zone which previously controlled the margin of the Silurian basin

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The Caledonides of the west of Ireland provide a well-exposed and well-mapped example of an oblique collision zone. The east-northeast trending Deer Park and Achill Beg Fault system is a crustal scale ductile sinistral strike-slip duplex of late Ordovician age, imbricating late Precambrian granulite facies lower crustal rocks, near eclogite facies supracrustal rocks, up to amphibolite facies Dalradian metasedimentary rocks and greenschist facies Cambro-Ordovician rocks. This fault system is correlated with a pre-

An interpretation of the major structure of parts of the Irish Caledonides is made, based on the results of mapping of the main early penetrative fabrics. Interpretation of major changes in fabric are attributed to strain variation, and models are developed to explain these changes. The regional nappe structures in the Dalradian rocks have been modified by (1) D 2 slide zones, and (2) a D 4 sinistral shear zone adjacent to the Main Donegal Granite in NW Donegal. In N Mayo the major structure is interpreted in terms of a steep dextral E-W shear zone modifying the early formed nappe pile. The strain history adjacent to the Iapetus suture in central Ireland is described and a wide zone of non-axial planar cleavage discussed. In SE Ireland the regional strain pattern is modified in the tectonic aureole of the Leinster Granite.