32-4 Voyages of Discovery and Survey. 



than at present prior to the entombment of the vegetation forming 

 the coal and the fossil trees (one of which, dug out at Christmas 

 Harbour, was seven feet in circumference). The coal of the same 

 harbour was in a horizontal bed, four feet thick, requiring no small 

 amount of vegetable matter to form it, and another coal-bed was 

 found in Cumberland Bay. Fossil wood is also noticed as scattered 

 through the igneous rocks. How far some of these rocks may be 

 consolidated ashes does not appear, but it might well happen, that 

 not only lava-currents may have flowed over a mass of vegetation, 

 perhaps sometimes a thick peat-bog, but also that a body of ashes 

 may have been vomited over them from a neighbouring crater. 

 Whether enveloped by ashes, subsequently consolidated, or by mol- 

 ten rock, the conditions for silicification of some of the wood remind 

 us of those in Tasmania. The prevalence of the remains of a vege- 

 tion now no longer found, is a fact of much geological interest ; for we 

 can scarcely doubt, from their mode of occurrence, that the plants 

 entombed grew on the spot. Here, therefoi'e, upon a small point of 

 land, projecting through the waters of the southern ocean, far remote 

 from continents (that of Victoria Land being probably the nearest), 

 we have evidence of changed conditions I'Cgarding the growth of 

 plants. The mere chance of such an investigation as could be given 

 affords the remains of a tree seven feet in circumferences, in a region 

 where small plants only can at present grow ; and we are left to infer 

 that when such trees flourished a milder climate reigned over this 

 land, now so desolate. 



Mr Beete Jukes has, in his Account of the Voyage of the " Fly" to 

 Torres' Straits and Australia, furnished us with much valuable in- 

 formation respecting that coral accumulation known as the Great 

 Barrier Reef, which extends for about 1000 miles in length, with 

 about 30 in mean breadth, from Breaksea Spit, off the eastern coast 

 of Australia, in lat. 24° 30' S., to Bristow Island, oft" the coast of 

 New Guinea, in lat. 9° 15' S. During the needful examinations by 

 Captain Blackwood, in command of the surveying expedition, Mr 

 Beete Jukes lost no apportunity of studying this interesting mass of 

 matter, due to the power of myriads of polyps to obtain from the sea 

 and secrete carbonate of lime. He divides the accumulation into, 

 \st, linear reefs, forming the outer edge or actual barrier ; 2d, de- 

 tached I'eefs, outside the main barrier ; and 3d!, inner reefs between 

 the shore and the barrier. The linear reefs vary from half a mile 

 to 15 miles in length ; the detached reefs take more or less the cir- 

 cular or oval form, with lagoons inside, to which Mr Darwin has as- 

 signed the name of atolls, and the outlines of the inner reefs are no- 

 ticed as of diftcrent shapes. On the outer side of this great mass of 

 coral accumulations, or of matter derived from them, the sea sud- 

 denly becomes deep, while on tlie inside it is comparatively shallow. 

 The edges gradually slope, or are rounded to the depth of 12 or 2-4 

 feet, after which tliey plunge with equal slopes suddenly into 120 to 



