Geological Society of London, 235 



the results of an investigation wMcli, he thought, ■would dispose of 

 all doubts on the subject. On cutting a trench through 1 foot 

 of peat and 1 4 inches of clay round one of the stumps, which had 

 an oak trunk lying by it, apparently in the position in which it had 

 fallen, the observers saw that roots were cut through all round, 

 running along near the surface of the clay, or penetrating it diagon- 

 ally ; while rootlets and tap roots descended vertically into the clay. 

 Several of the main roots were traced for a considerable distance 

 into the clay. On raising the stump out of the ground, the clay 

 showed numerous root-sections. The examination of the stumps 

 gave coniirmatory results. 



11.— April 3, 1878.— Henry Clifton Sorby, Esq., F.E.S., Presi- 

 dent, in the Chair. — The following communications were read : — 



1. " On an Unconformable Break at the base of the Cambrian 

 Eocks near Llanberis." By George Maw, Esq., F.L.S., F.G.S. 



In a paper read before the Society on December 5, 1877 (Q.J.G.S. 

 vol. xxxiv. p. 137), Prof Hughes referred to an observation made 

 by the author in 1867 as to the occurrence near Llanberis of an 

 Tinconformable break, indicating the base of the Cambrian ; and, 

 while accepting the asserted existence of pre-Cambrian rocks in 

 North Wales, placed the base of the Cambrian in a very different 

 position, and maintained that the appearances described by Mr. 

 Maw might be accounted for by lateral pressure acting upon beds of 

 dissimilar texture and unequal hardness. The author had re-ex- 

 amined the section in question, and maintained his original interpre- 

 tation of the phenomena, which he regarded as the earliest indication 

 of the existence of a pre-Cambrian series. He accounted for differ- 

 ences observed in the supposed pre-Cambrian rocks at Moel Tryfaen 

 and Llanberis by regarding them as having undergone different 

 degrees of metamorphism. 



2. " On the so-called Greenstones of Central and Eastern Corn- 

 wall." By J. Arthur Phillips, Esq., F.G.S. 



In this paper the author extended his investigations of the rocks 

 formerly mapped as greenstones, from the western (see Q. J. G. S. 

 vol. xxxii. p. 155) into the central and eastern districts of Cornwall. 

 He described in detail various rocks from different parts of these 

 districts, the examination of which had led him to the following 

 conclusions. The numerous lavas which occur here, in addition to 

 the rocks met with in Western Cornwall, are so interbedded with 

 the slates and schists as to lead to the conviction that they are con- 

 temporaneous, and, although much altered, they closely resemble 

 lavas of more modern date. Sometimes they assume a distinctly 

 schistose character. The crystalline greenstones are more varied 

 and instructive than those of the western portion of the county ; 

 some of them are typical dolerites, while others are so altered as to 

 consist only of a granular indefinite base, traversed by indistinct 

 microlitic bodies. Their pyroxenic constituent is augite ; and, 

 although many would call them diabases or melajDhyres, the author 

 regards it as more logical to regard them as ancient dolerites. 

 Where these rocks are altered, the augite is usually changed into 



