408 



THE GEOLOGIST. 



G. V. Du Noyer. " General Sketch cf the districts already visited by the 

 Geological SurVey of India/' by Prof. Tijomas Oldham, Superiureudent of the 

 Geological Survey of India. " On the Ironstone in the Oolitic district of York- 

 shire." by Prof. John Phillips. " On the discovery of Paradoxides in New 

 England," by Prof. W. B. Rogers. " On the Geological Survey of Feuusjlvania," 

 by Prof.' H. D. Rogers. " On the Fossils of the Dingle district," by J. W. 

 Salter. " On the Erosion of rivers in India," by H. and R. Schlagiutweit. 

 " On some facts connected with Slaty Cleavage," by H. C. Sorby. • 



This last communication drew attention to the microscopical structure of the so- 

 called clay-slates, as being entirely difierent from the modern deposit of clay, 

 formed from decomposed felspar, or from those occurring in strata which have 

 undergone no subsequent chemical change. Such, in fact, contain little or no 

 clay, in the usual acceptation of the term, but are often almost wholly composed 

 of very minute plates and crystals of a peculiar mica, so that they might be called 

 mica- clay- slates, thus being analogous to vei-y fine-grained mica-schists, into 

 which they pass by the increase in size of the crystals of mica. 



The form of these particles, and their manner of arrangement, are quite 

 dilferent from that exhibited by rocks merely deposited from suspension in mud, 

 but are extremely like what occur when minute crystals are formed in dtu. 



The opinion of the author is, that such clay-slate as is almost entirely com- 

 posed of mica was originally a deposit of ordinary felspar- clay, and that, probably 

 under the action of water at a high temperature, this was altered into a mass of 

 minute crystals of mica. This view is of interest as explaining the promiscuous 

 arrangement of the minute particles of mica before the cleavage was de ^ eloped by 

 pressure. 



Two distinct kinds of slaty-cleavage were then pointed out, hitherto confounded 

 under the term slaty-cleavage. One of these, characteristic of the best roofing- 

 siates of V/ales, the author styles ultimate-stnictare-cleavage, as being the structure 

 which would result from the rock yielding to pressure as a plastic substance ; the 

 amount of absolute compression when cleavage was developed, as deduced from 

 the green spots, indicating the amount of water squeezed out as equal to the 

 quantity required to render the clay quite plastic. 



The other extreme structure is a cleavage due to very close joints, often 

 undistinguishable except by the aid of the microscope ; whilst the arrangement 

 of the particles in the spaces between them is independent of the joints, and is often 

 related to quite another plane. This kind the author styles close-joints-cleavage, 

 and it corresponds with the result of the rock yielding to change of dimensions, 

 like a rigid body, by the formation of close cracks. 



As these two kinds of cleavage obey materially different laws, the structure 

 affords an indication of the actual condition of the rocks at the time of their 

 compression, and perhaps, also, in some cases, whether the movements of elevation 

 were sudden or gradual. 



" On a Fossil antler of a Stag from the Severn Drift," and " On a New Species 

 of Eurypterus from the Old Red Sandstone of Herefordshire," by the Rev. W. S. 

 Symonds. " On the Geology of the Galty Mountains," and " On the Tertiary Clay 

 and Lignite of Ballymacadam, near Caher, County Tipperary," by k. B. Wynne, 

 of the Geological Survey. 



In the Botanical Section there were also papers of interest to the geologist, 

 especially one by Mr. D. Moore, on the plants which, by their growth and 

 decomposition, form the principal part of the Irish turf-bogs. 



The Rev. G. O'Meara also gave a notice of some forms of DiatomacetB found in 

 the chalk of Antrim, having the curious result that the majority of 42 forms 

 recognised appear to be freshwater diatoms existing at the present day. 



