14. EPIDOTISED PORPHYRITIC MONZONITE. In the hand specimen this is a black and yellow rock in which porphyritic crystals of felspar up to nearly 10 mm. length are easily seen. The slice shows that the rock consists of euhedral zoned plagioclase crystals with decomposed cores, perhaps originally labradorite, with oligoclase margins, set in a matrix of fresh orthoclase and subordinate quartz. Hornblende is common, and is associated with chlorite after biotite and a little ilmenite. Epidote occurs throughout the rock. Apatite is accessory.
   15. QUARTZ DIORITE. Medium-grained rock, with grain size up to about 1·5 mm. Quartz is common, but rather local, in the slide, and the felspar is mainly andesine with some orthoclase. Hornblende is abundant, but is much 

associated with chlorite and iron ores, and may be decomposed augite. Accessory minerals are limonite and apatite.
   Most of the rocks in this collection resemble types known in the Drift of East Anglia, but the latter have not yet been worked out in sufficient detail to say definitely whether these Stonar erratics might have come from there. It should be pointed out that in any case they may also have been derived from the Thames Plateau and River Gravels, but it would not be easy to distinguish between the Thames erratics and those of East Anglia, because in some cases these two groups have been derived from a common source.