218 



THE GEOLOGIST. 



fossils only : the chalk itself is of a creaiiiv colour, and the (b"awings on it look as 

 if done with ludiiin ink by a very fine pencil. I have observed the same descrip- 

 tion of markings on the chalk and fossils of the White Rocks, near Duiduce 

 Castle, Coimtv'Anti'im. 



" I send you a small quantity of the dendi'itic gi-ains, and a little hit of chalk. 

 The di-aTving upon it is not so distinct as I could wish ; m some chalk it is per- 

 fectlv black,"and very l.->eautiful indeed, and in aU such instances I observe that the 

 chalk is of peculiar hardness, while at the same time the markings penetrate 

 deeper, with increased depth of cclcau' as they proceed : but in the softer chalk the 

 tintings are brownish, and mere i^n rlie surface. 



" Are those moss-Kke ri^ure'? c; rnm-ii to all chalk ? 



" 2. Why do tliose portions of Pentacruiites. commoiily called *■ star-stones,' 

 move with a sensible rotatoiy motion in a vessel of nnegar { or why do they move 

 at all / And why will n-t a portion of Belemnite, or plain limestone, both from 

 the same rock, and of t]ie same size as the ' star-stone,' move also ] — Very truly 

 yom-s, A. DE S, M.. Poit Stewart." 



1st. The beautiful feathery dendritic mai'kings in clialk and on chalk fossils are 

 due to a ciystaRization of manganese either on the planes of cracks and fissm-es, 

 in the fine interstices lietweeu the rock-matrix and the fossils, or filling up minute 

 cracks in the structure of the rock itself. Such dendiitic markings are found in most 

 sohd rc>cks, and have been veiy commonly attributed to u'on. We beheve, however, 

 that when ii'on puts on the dendiitic form it is due to the admixture -witli it of 

 manganese. Where the propoiti'3n of iron is larger, the dendiitic markings are 

 dingy and obsciu'e, and by a few days" exi'osure to the au' diminish to a brown 

 stain. Wliere the manganese predcauiuates, the dendiitic ornamentation is shaii>, 

 clear. ' ' ;ed, and of a dense black eoloiu'. 



W'. .lysed the sample foi-wai'ded, and find that it contains the lai-ger 



propjiiivn lI rnanganese with traces of iron. 



The quantity sent wa.s quite sulficient for a qualitative, but not for a quanti- 

 tative, analysis'; so that we cannot state the exact proportion of the two ingie- 

 dients. These dendritic mai'kuigs therefore are due to manganese. 



VeiT pretty dendiitic sciilptmings are to be made artificiaily, by mixing clay with 

 a solution of" sulphate of ojpper, and bakmg or otherwise quickly drying the' mass. 



2d. The " Star->t:nes move on account of the evolution "(by" the action of 

 the vinegar upi'ii the earljonate of lime of which the pentaciinite plates are com- 

 posed; c"f small gl-l 'iiles of carbonic acid gas. Sometimes these may be seen 

 clustered rotmd the ^tar-stone," but often they are so minute, fi-om the very slow 

 action of the ^inegai". a^ to be scarcely visible with a pocket-lens. 



Why bits of Inuestone or l>elemniies do not likewise move, may be thus 

 explauied : — 



The star-stones are of veiy hard and compact stnictm-e, and the gas therefore 

 is very slowly evolved by the weak action of the vinegai'. From this cause the 

 minute bubbles congTegate under the star-stone, where they ai-e, by the hoUows 

 and sculptm'mgs of which they are retained, and the star-stone, rendered slightly 

 buoyant. Then as the buljl.iles biii*st into each other, and become confluent, or as 

 they tilt up fii^st one edge then another of the star- stone in escaping, a shght 

 mechanical action is set up sufficient to cause tlie r tat jiy motion aUiided to by 

 oiu' coiTespondent. In the case of common belemiiitcs. limestone, &c., the cai'bonate 

 of lime being softer, the gas is generated more quickly and in larger bubbles, 

 and consequently readily escapes "without at all ser\-ing to float, by clinging to 

 it, the object from which it emanates. 



*' Dear Sir, — I am frequently apphed to by stranger geologists, respectmg 

 infonnation on the Ledbmy and Malvern districts. You woidd, I think, be 

 rendering ramblers a ser^ice'by informing them that they will find Henry Brooks, 

 shoemaker, the Homend, Ledbiuy, a raost uitelhgent and efficient giiide. He 

 hiis lately discovered there ' Auchcnaspis Salferrii,' one of tlie rarest of the Tilestone 

 fossils.— Yours very truly, ^Y. S. Stjio^^ds, Pendock Rectoiy, Ledbmy." 



The great IcHTHYOSAURrs platyodox (?) at the York Museum. — "Sir, — 

 I w;\s glad to see your con-esponilent, 3Ir. S. R. Pattisc-n, bringmg forward the sul\iect 



