704 THANSACtlONS Ot' SBCtIoK C. 



10. The Tourmaline Rocks of Cwm Dwythwc, near Llanberis {North 

 Wales). By W. G. Fearnsides, M.A. 



Some years ago, when examining sands from the neighbourhood of 

 Caernarfon, I found that both the river sands of the Seiont and the beach sands of 

 the Menai contain tourmaline. In order to trace the mineral to its source I have 

 since examined the heavy mineral residues of the sands of nearly all the tribu- 

 taries of the Seiont, and find that all those which flow across the Cambrian Slate 

 Belt contain either needles or broken grains of brown tourmaline. The sand from 

 the Afon Arddu (whose delta parts the two lakes of Llanberis) is exceptional, and 

 is verj' rich in well-formed trigonal prisms of blue tourmaline. The sand from the 

 Afon Hwch, its tributary from between Moel Eilio and Moel Goch, is even more 

 surprising, and in the sand from the spits along the flatter reaches of this burn 

 tourmaline can generally be distinguished with a poclcet lens. 



I have therefore mapped the Cwm Dwythwc on the 6-inch scale, and in 

 mapping have found the tourmaline rocks in situ. They are mostly coarse grits, 

 grits, flags, and slaty flags, and occur along the horizon of the unconformity 

 between Cambrian and Ordovician rocks. The tQurmaline is not clastic but lias 

 been formed in situ from the feldspathic ground-paste of the grits or flags, ar.d 

 clustered new-formed needles enter and pierce the quartz pebbles of the grit or 

 the chloritoid ground mass of the slate in a most fascinating manner. There 

 has been much thrust-faulting along the unconformity, but no large intrusive 

 mass of igneous rock has been observed within five miles of the locality. Tour- 

 maline new-formed in the slate and the remains of tuning-fork graptolites can 

 be found within three or four inches of each other. The tourmaline is a soda- 

 bearing variety. 



11. Note on the Occurrence of {so-called) Cave Pearls. 

 By Harold Brodrick, M.A. 



Cave pearls, as they were first called in ' Cave Hunting ' CProfessor Boyd 

 Dawk ins), seem to be of comparatively rare occurrence. They consist of a nucleus 

 of some foreign material, frequently a small pebble of Yoredale rock (in one ease a 

 small fragment of lead ore) coated by numerous concentric rings of calcite. All those • 

 that I have found have been formed under similar conditions; they have all been 

 found in what might be called nests in the rock, into which drops of water have 

 fallen at comparatively long intervals from a considerable height. Each falling 

 drop will have the tendency slightly to turn the nucleus, and also, by deposition, 

 to coat it with a thin film of carbonate of calcium ; this depo.^ition is continued 

 until what is called a cave pearl is formed, ranging in diameter from 0-5 cm. up 

 to 2 cm. 



The three types with which I am acquainted come from three separate caves : 

 (1) The Blue John (Derbyshire). This type consists of a nucleus of Yoredale 

 Sandstone covered with layers of calcite, which become harder towards the out- 

 side, the exterior being extremely hard, smooth, and opaque ; sp. gr. 2*75. (2) The 

 Bagshawe (Derbyshire). This type consists in many cases of a nucleus of Yoredale 

 Sandstone, or, in one case at least, of lead ore ; the concentric deposit in this type 

 is somewhat translucent, the outer surface is slightl}- crystalline; sp. gr. 2-71. 

 (3) Marble Arch (Co. Fermanagh). This type has a nucleus of Yoredale Sand- 

 stone, while the covering is composed of carbonate which seems to have included 

 in it a considerable admixture of contained mud ; the colour is a dirty grey, and 

 the deposit is comparatively soft; sp. gr. 2-40. As will be seen from the specific 

 gravity in each case, the deposit is in the form of calcite, a condition which might 

 be anticipated from the mode of occurrence. I have carefully examined active 

 streams in numerous caves for similar formations, but have been unable to find 

 them. The caves in which they have been found (to my knowledge) are Caldy 

 Island (Boyd Dawkins), the Blue John Cave (two nests), the Bagshawe Cave (one 

 nest), about thirty pearls; Marbte Arch Cave (one nest), at least a hundred 

 pearls. 



