Page:A sketch of the physical structure of Australia.djvu/43

31 porphyritic greenstone, Turtle-backed Island was nearly all granite except its N.W. corner, which was a brown and liver coloured laminated quartz rock. Cap Island is composed of sienite, and from its shape I concluded Mount Cornwallis was also granitic. Around these islands in this north and south central band of Torres Straits, the depth of the sea is very uniform being from eight to ten fathoms, the bottom being nearly everywhere mud. On the N.W. side of it are some shoals said to be coral reefs, rising apparently from no great depth of water. The whole of the eastern part however of Torres Straits is occupied by true coral reefs forming the northern part of the great barrier reefs. The islands here with three exceptions are all flat coral islets, rising not more than twenty or thirty feet above high water mark, the water round them gradually deepening to the eastward. The exceptions are the groups of the Murray Islands, Erroob or Darnley Island, and some rocks on Caedha or Bramble Key. The rocks in these three localities are volcanic, consisting partly of sandstone and conglomerate made of pebbles of lava and coral limestone, with some beds of finer tuff, and partly of large masses of dark heavy hornblendic lava. The eruption of these volcanic rocks though probably of comparatively modern origin, geologically