403 
THE GEOLOGIST. 
0. V. Du Noyer. "General Sketch of the- (li^tl■icts nlrpiuly visitril by the 
Geological Survey of India,'' by Trof. Thomas Oidham, Superintendent of the 
Geological Survey of India. " On the Ironstone in the Oolitic district of Yoik- 
shire," by Prof. Joliu Phillips. "On the discovery of Paradoxidts in JS'ew 
England,'' by Prof. W. B. Rogers. " On the Geological Survey of Penn.sylvania," 
by Prof. II. D. Rogers. "On the Fossils of the Dingle district," by J. W. 
Salter, " On the Erosion of rivers in India," by II and 11. Schlagintweit. 
" On some facts connected with Slaty Cleavage," by H. C. Sorly. 
This last communication drew attention to the luieroscoj.ical structure of theso- 
calicd clay-slates, as being entirely dillert nt from the modern dopi.sit of cl.ay, 
formed from decoinposed 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-clivy-slates, thus being analogous to very fine grained mica-schists, into 
which they pass by the iucrea.^ein size of the crystals of mica. • 
The foim of these particles, and tLeir manner of arrangement, are quite 
different from that exhibited by rocks merely deposited from suspension iu mud, 
but are extremely like what occur when minute crystals are formed in iilu. 
The opinion of the author is, that such clay-slate as is almost entirely com- 
posed of mica was originally a deposit of ordinary ftlspar-clay, and that, probably 
under the action of water at a high temperature, this was altered intu a mass of 
minute crystals of mica. This view is of interest as explaining the pr uniscuoua 
arrangement of the minute particles of mica before the cleavage wa- developed by 
pressure. 
Two distinct kinds of slaty-cleavage were then pointed out, hitherto confounded 
under the term slaty-cleavage. One ot theie, tliuracteristic of tlie best roofing- 
slates of Wales, the author stj'les ulliiiiate-slruclnre-clecwaf/e, as being the structure 
which would result from the rock yielding to pressure as a plastic substance ; tho 
amount of absolute compression whon cleavage was developed, as deduced from 
the green spots, indicating the amount of water squeezed out as equal to the 
quantity lequired to render the clay quite plastic. 
Tiie 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 dose-joint s-ckac(if/e, 
and it corresponds with the result of the rock yielding to change of dimensions, 
like ii rigid body, by the formation of close ciacks. 
As these two kinds of cleavage obej' materially different laws, the structui'S 
affords an indication of the actual condition of the I'ocks 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 Herefordsiiire," by the Rev. W. S. 
Symoiids. " On the Geology of the Galty i\Iountaius," and " On tlie 'i'ertiary Clay 
and Lignite of Ballymacadam, near Caher, County Tippcrary," by A. 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 Diatomacere 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. 
