﻿Geological Society of London. 185 



of moderate thickness, elevations such as suggested in the Address might -well suffice 

 to bring about a shifting of the crust, either by sliding on the fluid or viscous interior, 

 or by causing it to undergo a certain amount of gradual deformation. The thinner the 

 crust, provided it were sufficiently rigid to support the elevations once made upon it, 

 the more readily would its geographical position be changed with regard to the poles. 

 With regard to the thickness of the crust at the present time, he did not despair of 

 astronomers at last conceding a less thickness than that assigned by the late Mr. 

 Hopkins and Sir William Thomson. He was glad to find that the latter, in his 

 Address to- the Mathematical Section of the British Association at Glasgow, was 

 willing not merely to admit, but to assert as highly probable, that the axis of 

 maximum inertia of the earth and the axis of rotation, always very near one another, 

 may have been in ancient times very far from their present geographical position, and 

 may have gradually shifted through 10, 20, 30, 40, or more degrees, without at any 

 time any perceptible sadden disturbance of either land or water. 



Mr. George Darwin, also no mean mathematician, in his paper recently communi- 

 cated to the Eoyal Society, agrees as to the probability of large geological changes 

 affecting the position of the poles, and regards the effect of such changes as possibly 

 cumulative. 



Mr. Evans felt that the Society was much indebted to Mr. Twisden for having 

 likewise investigated the question, in which, of course, he was personally much 

 interested. 



Prof. Ramsay referred to the former prevalence in Geology of views which had 

 long since been exploded, and expressed his opinion that before long the theory of 

 the fixed geographical position of the poles of the earth would share the same fate, 

 and that the position of the poles would be regarded as very variable indeed. The 

 Flora of various deposits in Polar lands indicated the groAvth of plants which would 

 require the stimulus of light, even if the necessary amount of heat for their growth 

 could be accounted for. With respect to great local changes of level, he remarked 

 that the northern part of Africa was 4000 feet below the level of the sea in very 

 recent times, and there was considerable reason to suppose that even later than the 

 Miocene a vast tract of land occupied the space between what were now the con- 

 tinents of Africa and India. Of course no one supposed that the position of the 

 poles had been changed by rapid upheavals of land; but as we know that all geo- 

 logical changes of level have been slow and gradual, so the poles may have altered 

 their position by a process as slow as that of evolution which has originated the 

 species of animals and plants during the long series of geological time. Such 

 questions as these were serious questions involving the results of much observation, 

 and could not, he thought, be solved in the closet by any amount of geological work. 



Prof. T. McKenny Hughes, Mr. A. W. Waters, the Rev. J. F. Blake, and Prof. 

 Seeley, also took part in the discussion, and the author briefly replied. 



2. " Note on a Specimen of Biploxylon, from the Coal-formation of Nova Scotia." 

 By J. W. Dawson, LL.D., F.R.S., F.G.S. 



The author described the occurrence in Coal-measure sandstone at the South 

 Joggins of an erect stump of a Sigillarian tree 12 feet in length. It originated 

 in a coaly seam 6 inches thick, and terminated below in spreading roots ; below the 

 coal-seam was an under-clay 3 feet 4 inches thick, separating it from an underlying 

 seam of coarse coal. The stem, which tapered from about 1\ feet in diameter near 

 the base to IJ foot at the broken end, was a sandstone cast, and exhibited an internal 

 axis about 2 inches in diameter, consisting of a central pith cylinder, replaced by 

 sandstone, about f inch in diameter, and of two concentric coats of scalariform tissue, 

 the inner one ^ inch in thickness, the outer constituting the remainder of the axis. 

 The scalariform tissue of the latter was radially arranged, with the individual cells 

 quadrangular in cross section. A few small radiating spaces partially filled with 

 pyrites obscurely represented the medullary rays, which were but feebly developed ; 

 the radiating bundles, passing to the leaves, ran nearly horizontally, but their structure 

 was very imperfectly preserved. The cross section, when weathered, showed about 

 twenty concentric rings; but these under the microscope appeared rather to be bands 

 of compressed tissue than true lines of growth. The thick inner bark was replaced 

 by sandstone, and the outer bark represented by structureless coal. On a small 

 portion of one of the roots the author traced the remains of stigmarioid markings. 

 From the above characters the author identified this tree with Diploxylon of Coraa, 

 and stated that it was the first well-characterized example of this type of Sigillarians 



