THE LOWER MARGIN OF THE SHIANT ISLES SILL

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Abstract
At one locality near or at the lower margin of the main Shiant Isles sill there are contacts with Jurassic marl. Immediately adjacent to the highest of these contacts there is clear evidence of an endogenous reaction—Mg-olivine→ titanaugite. Zeolites tend to be concentrated near the contacts with marl and there is some evidence that teschenitic rock can result from exogenous reactions within the marl. An unusual degree of variability in the textural relations of olivine and augite with plagioclase is described and discussed. It is suggested that where there has been assimilation of marl, augite may have been precipitated instead, or as a replacement, of early olivine and that rocks not readily distinguishable from alkaline dolerites and teschenites were produced on a limited scale by interaction with the marl of water-and soda-rich picritic material intruded slightly in advance of the main picritic facies of this sill. Interbanding of picritic and teschenitic rocks at another locality can be interpreted as due to flowage and intrusion of these products of mixed origin. Analogous phenomena in some other sills are briefly discussed.

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