460 Geological Society. 



GEOLOGICAL SOCIETY. 



May 6. — A paper was read entitled " A brief description 

 of an extensive hollow or fissure, recently discovered at the 

 quarries near the extremity of the Western Hoe, Plymouth ;" 

 by the Rev. Richard Hennah. 



In this communication the author describes an extensive 

 hollow or cave in the limestone rocks near Plymouth, in which 

 no remarkable bones have yet been discovered, but in which 

 stalactites are particularly abundant. Mr. Hennah offers some 

 remarks on the various causes and circumstances which have 

 contributed to give to these stalactites their different shapes 

 and compositions. 



A paper entitled " On a dyke of serpentine, cutting through 

 sandstone in the county of Forfar;" by Charles Lyell, Esq. 

 Sec. G.S. was read in part. 



May 20. — The above paper was concluded. 



In the former part of this paper, the rocks which are ex- 

 posed on the left bank of the Carity, a small river in Forfar- 

 shire which descends from the mica-schist district of the 

 Grampians into Strathmore, are described. The first of these 

 is a clay-stone porphyry ; next to it is a conglomerate contain- 

 ing quartz pebbles; and then strata of fine-grained micaceous 

 sandstone and shale dipping to the south, and which are sud- 

 denly cut off at an angle by the serpentine. These strata of 

 sandstone and shale form part of a great series which overlies 

 the clay-slate, to which it immediately succeeds ; and is older 

 than the great conglomerate of the old red sandstone, which 

 hes immediately upon it. The serpentine is vertical, and is 

 well characterized. It contains in part A'eins of asbestos, and 

 in parts diallage, and a large mass of hypersthene. On the 

 other side of this dyke of serpentine, which is 90 yards thick, 

 fine-grained sandstone and conglomerate again appear, and 

 slip away from the serpentine towards the south. Next to these, 

 a mass of serpentine is seen mixed with dolomite, and at its 

 side altered sandstone and a conglomerate in which the quartz 

 pebbles are split and re-united by ferruginous matter. Lastly, 

 at a short distance a dyke of greenstone parallel to the serpen- 

 tine occurs, flanked on both sides by vertical masses of sand- 

 stone and conglomerate much altered and indurated, and 

 charged with brown spar. 



Mr. Lyell next describes the I'ocks on the right bank of the 

 river, which resemble those on the left, with one exception, viz. 

 that the great dyke of serpentine seems to be connected with 

 the mass of dolomitic serpentine, a thin bed of fine-gi'ained 

 greenstone alone intervening, and the sandstone and conglo- 

 merate, 



