Carruthers — On British Graptolites, 125 



tinuous ; the limits of many cannot be traced, but tbey also occur as 

 isolated patches environed on all sides by the slate. The nuclei of 

 the bleached spheres represent their smallest development, between 

 which and the continuous layers patches of every intermediate size 

 occur. 



The ultimate composition of the green beds would be consistent 

 with their containing a large proportion of hornblende. Bischof^ 

 refers to beds of similar character interstratified with clay slates in 

 the Thuringian Forest, and discusses the question whether they may 

 not be a metamorphic product from the slates ; but in the case of the 

 Welsh slates, whatever be the source of the materials composing the 

 green layers, whether simply detrital accumulations from eruptive 

 rocks, or otherwise, their sedimentary interstratification is scarcely 

 open to question. 



The bleaching of the blue slate adjacent to the dykes of intrusive 

 Greenstone seems to be of a different character to that adjacent to 

 the interstratified layers. Instead of being defined by a distinct out- 

 line as in Plate VII., the pale colour graduates into the normal 

 colour of the slate, and analyses of examples from the Penrhyn and 

 Llanberis quarries indicated that the change of colour was due to 

 the reduction of most of the iron from sesquioxide to protoxide, 

 probably by the agency of a moderate degree of heat on the in- 

 trusion of the Greenstone. As I believe there is no published 

 analysis of the composition of these Welsh Greenstones, it may not 

 be out of place to give the result of a determination made for me by 

 Dr. Voelcker for comparison with the composition of the green 

 layers interbedded with the slate. 



Analysis op Greenstone Dyke, Penrhyn Quarries. 



"Water of Combination 1*99 * Carbonate of Lime 14-85 



Bisulphide of Iron 0'23 *Carbonate of Magnesia 14-59 



Protoxide of Iron 10-22 Potash 0-43 



Peroxide of Iron 1-97 Soda 0*70 



Titanic Acid 2-51 Silica 47-47 



Alumina 5-80 



Sulphate of Lime 008 100-633 



* The carbonate of lime (and magnesia ?) occurs for the greater part in the shape 

 of separate crystals, which are visible in a fresh fracture to the naked eye, and 

 eflfervesce in isolated spots on the application of hydrochloric acid. 



IV. — A Kevision of the British Graptolites, with Descriptions 



OF THE New Species, and Notes on their Affinities.'^ 



By Wm. Carruthers, F.L.S., F.G.S., Botanical Department, British Museum. 



(PLATE V. p. 64.) 



Class, Hydrozoa. Order, Graptolitidce. 



Gen. I. Rastrites, Barr. (Grapt. deBoh. p. 64). Polypary simple, 

 consisting of a slender capillary common tube, supporting a single 

 series of isolated hydrothecae, which are free throughout their whole 

 length. 



^ " Chemical and Physical Geology." English edition, vol. iii., p. 315. 



' Concluded from the February Number of the Geological Magazine, p. 74. 



