Discussion.

Dr. Hicks, having studied the adjoining rocks of the mainland of Scotland, differed from the authors as to the age of the metamorphic scries, and thought they must be Pre-Cambrian, and belonging to two if not three Pre-Cambrian series.

Prof. Bonnet asked as to the evidence of the passage of gabbro into serpentine.

Mr. Horne replied that they did not attempt in the paper to fix the age of the metamorphic rocks referred to by Mr. Hicks, and insisted on their views as to the intrusive character of the quartz-felsites of Shetland.

In reply to Prof. Bonney, he stated that he and his fellow author had not minutely studied the supposed passage of one rock into the other, but that Dr. Heddle, who had so studied them, had arrived at the same conclusion as themselves.

Quart. Journ . Geol. Soc .Vol. XXXV. Pl. XXXIX.