84» Prof. Brongniart on Websterite^ fyc. 



mation. At Mori also, not far from the last-mentioned spot, 

 we meet with a similar substance, determined by M. Stro- 

 meyer to contain the same principles as that of Halle. 



We have next to consider a new variety of this mineral, 

 which is met with at Auteuil, and which is properly the sub- 

 ject of this memoir. 



The chalk which supports all the superior sedimentary 

 strata in the basin of Paris is seen uncovered at Meudon, but 

 does not appear on the right bank of the Seine ; it is however 

 very near the surface under the hills called Point-du-Jour ; 

 and also near Auteuil it carries the plastic clay, which covers 

 it in many points. Indeed, this clay is worked at the foot of 

 the village of Auteuil, in a place called la Glaciere, and is used 

 for making bricks and other purposes. It was in this spot, or 

 very near it, that M. Becquerel found in the plastic clay, lig- 

 nite, pyrites, sulphate of strontian, phosphate of lime, and 

 even a little blende : there also are procured the large well- 

 defined and clear crystals of gypsum, so much prized by ama- 

 teurs of fine minerals. This locality, in the part where I have 

 examined it, exhibits, immediately under the vegetable soil, a 

 plastic clay yellowish and very sandy, having little tenacity, 

 traversed by veins of yellowish clay still more sandy, ochrey, 

 and divided into many small portions. Beneath, is a blueish 

 plastic clay that is more tenacious, and contains a good deal 

 of pyrites and gypsum. Still lower, is a bed of sand, or rather 

 yellow ferruginous coarse gravel ; and below that another bed 

 of clay. In this upper yellowish clay we find, and rather in 

 nodules than in veins, some parts that are whitish and friable, 

 composed of a multitude of small roundish grains, closely con- 

 nected together, yet admitting a greyish clay in the interstices. 

 When these little masses are cut across, they present the aspect 

 of an oolite consisting of white grains in a greyish paste or 

 cement. This is Websterite. Each grain, when closely ex- 

 amined, appears to be a little spheroid of a structure indi- 

 stinctly radiated. When crushed, the powder is very brilliant 

 and soft to the touch ; and when examined by the microscope, 

 exhibits wedge-shaped masses composed of prisms, but so ill- 

 determined, that it is necessary to be aware of their crystal- 

 line structure to recognize them. 



It is from these characters that I conjectured this white 

 oolitic substance to be Websterite ; but the complete chemical 

 analysis made by M. Dumas leaves no doubt as to its nature. 

 It does not effervesce with nitric acid, which proves that the 

 argillaceous interposed part is not marl, but plastic clay. 

 When heated in a glass tube, water rises at first ; but when 

 the tube becomes red hot, sulphurous acid is disengaged in 



great 



