SCIENCE- GOSSIP. 



337 



the plane of the stratum in question, unless such 

 stratum still retains the horizontal position. 



In tracing northward the outcrop of the Gault, as it 

 appears on the west of the line of chalk hills known 

 as the Chiltern Hills, Gog Magog Hills, and the 

 Norfolk Wolds, it commences to decrease in thick- 

 ness, developing in its upper part beds of a calcareous 

 nature. When again looked for in the north of 

 Norfolk, it has lost its clay character, and, instead, 

 appears as what has been called Red Chalk, or 

 Hunstanton Limestone, a Chalk of a reddish hue, 

 and still containing a large proportion of clay. 



There is no doubt now that the greater part of the 

 Upper Greensand was formed contemporaneously 

 with at least the upper portions of the Gault clay in 

 other districts. For this reason it is impossible to 

 deal with the two as distinct formations, although 

 they differ so greatly lithologically. The Gault itself 

 is as a rule easily recognisable, but not so the Upper 

 Greensand, which varies greatly in different parts, 

 being sometimes a sandy marl, at other times a true 

 Greensand, containing glauconitic grains, whilst 

 sometimes the sands are coloured yellow and grey, 

 and consist chiefly of quartz and mica. As we watch 

 the outcrop of the formation from beneath the Chalk 

 ■on the west of the chalk hills which extend from 

 Salisbury Plain to Norfolk, we see the character of 

 the rocks gradually alter, assuming a more and more 

 clayey nature, until it also appears finally to pass into 

 the red clayey-limestone of Hunstanton before re- 

 ferred to. 



Red Chalk is also developed in Yorkshire, and 

 derives its colour from containing about 4 per cent, 

 of oxide of iron. When treated with acids it leaves 

 a distinct argillaceous residue behind, consisting of 

 alumina and oxide of iron, with a small proportion 

 of magnesia and potash. In composition it seems 

 to be comparable in some respects with the Red 

 Clays which have been referred to in discussing the 

 question of deep-sea deposits. 



The Upper Greensand occurs uniformly above 

 the Gault around the Wealden area, although it is 

 wanting in Mid-Kent. Together with the Gault it 

 has taken part in those great earth-movements which 

 folded the Weald into its present conformation, and 

 which probably affect all the underlying rocks down 

 to the denuded Carboniferous land surface. It is of 

 no great thickness, varying from 3 feet to 130 feet, 

 and often is unrecognisable as a sand. After the 

 deposition of the Gault there was apparently a 

 gradual upheaval of the sea-bottom, the Upper 

 Greensand being deposited in shallower waters, and 

 being derived from the waste of not very far distant 

 land. Around the Weald there was little or no 

 disturbing element to the equable deposition of this 

 series, but during the same period the more westerly 

 earth-movements appear to have been such as to give 

 rise to strata of more varied constitution. It is a 

 remarkable fact that the Gault rests at Ware, as 

 shown by a boring, immediately upon Silurian strata, 

 whilst at Cheshunt (Tumford) there is nothing be- 



tween it and strata of Devonian age. So that 

 during great part of both Primary and Secondary 

 epochs these spots were apparently dry land, be- 

 coming at last submerged beneath the Gault sea. 

 When a change again came and the sea became 

 shallower the sea-deposit was of a different nature, 

 and with the rise of Primary rocks somewhere in the 

 neighbourhood these became denuded, and their 

 "derived" fossils were mixed up with the new 

 shallow-water deposit there forming. 



( To be continued. ) 



BRITISH FRESHWATER MITES. 



By Charles D. Soar, F.R.M.S. 



{Continued from page 303. ) 



GENUS HYDRACHNA MULLER. 



THE name Hydrachna was given by Miiller in 

 1781 to all the water miles described by him ; 

 but Koch divided the family into a number of genera, 

 which have since been much increased, thus leaving 

 only a comparatively small number of species in the 

 genus Hydrachna. It contains, as far as I have 

 been able to ascertain, about thirty-three species : 



Fig. i. H.globos*. Ventral surface. 



but at present I have very lew to report from 

 Britain. Piersig records seventeen for Germany. 



The characteristics of this gemis arc: Body soft- 

 skinned, swimming hairs to all legs, mouth organ 

 extending as far forward as the palpi. 



1. Hydrachna globosa de Goer, i/7 8 - 



Body. — Nearly circular in form, and very thick, 

 and, as its name suggests, globular. Length about 

 2.30 mm., width about 2.15 mm. Colour a Lriyli: 

 red. Eyes rather close together. Behind each eye 

 is a chitinous plate (fig. 2), which is the principal 

 point of identity in this species. Between these 

 two dorsal plates is what appears to be another 



