MINERALOGY. 



423 



Geognoir. the nature of clay-slat*, which is often highly impreg- 

 **~~Y~* m ' ' nated with silica, thus giving to the mass a considerable 

 degree of hardness. The imbedded portions vary in 

 size, but ieldom exceed a few inches in breadth and 

 thickness. When the imbedded portions become very 

 small, the rock assumes a slaty structure, and forms the 

 Grey-nacke Halt of geognosti. When the grains al- 

 most entirely disappear, and the rock is principally 

 composed of clay-slate, it is named transition clau-slalc. 

 clay-slate has frequently a much more earthy as- 

 pect than the varieties found in the primitive districts. 

 Besides the grey-wacke already described, which has 

 the conglomerated structure, another having the same 

 structure is occasionally met with in transition districts. 

 It lias been named transition conglomerate, or pudding- 

 tlone. It is composed of roundish or angular masses 

 of granite, porphyry, gneiss, and clay- slate, often larger 

 than a man's head, imbedded in clay slate, or nearly 

 without a basis or ground. 



Common grey-wacke does not occur so frequently as 

 greywacke slate and transition clay-slate. 



Subordi- Subordinate bedt. Transition clay-slate, and grey- 



Bate bed*, wacke, contain occasionally different kinds of rocks in 

 subordinate beds, or in veins. The following are the 

 principal of these : 

 Quartz. 1. Quartz. It occurs in beds, imbedded masse*, and 



veins, and frequently in very considerable quantity. 

 Tak. 9. Talc.l his mineral occur* in imbedded iiiann. 



and in layers in transition clay-slate. There is a fine 

 example of it in the clay-slate of Claris, in Switzerland, 

 which is remarkable for the petrified fishes it con- 



the nucleus of the terebralulilct MHMrntf and paradox- Gognoy- 

 vs. It sometimes also contains turbmilei ana catnitet. ''""Y" 1 * ' 

 But the rarest and most interesting of the animal re- 

 mains which occur in transition clay-slate, are those of 

 Juhes, of which there is an instance at Plattenberg, two 

 leagues south-east of Claris, in Switzerland. This 

 fact is stated by several geologists, but we are of opi- 

 nion, that it has not been fully proved that the rocks 

 of Plattenberg are truly transition. 



Mctalliferout Minerals. Ores of various descriptions, Mtullifc- 

 abonnd in grey-wacke and transition clay- slate; thus roui mine- 

 the lead mines of Leadhills, and of Wanlockhead, are "^ 

 situated in these rocks, and the same is the case with 

 the productive lead 'and silver mines of the Hartz in 

 Hanover, of Vorespotack in Transylvania, of Brittany 

 in France, and of Guanajuato and Zacatecas in Mexi- 

 co. 



Geographical Distribution. Grey-wacke and transi- Geographi- 

 tion clay- slate abound in all the mountain ranges to ' ditri- 

 the south of the Frith of Forth, are also frequent to bulion - 

 the north of the same boundary, and are widely distri- 

 buted in England. On the continent of Europe they 

 form a principal feature in the mountains of the Hartz, 

 extend through Switzerland and the Pyrennees, and oc- 

 cur in vast abundance both in North and in South 

 America. 



'Wbtt tlau. 3. Wket tlate. Th'a mineral forms beds in the clay- 

 slate. 



4. Serpentine. Beds of serpentine, often of great 

 thickness, and of considerable extent, occur along with 

 transition clay -slate. 



5. Lydian it one. Beds of this rock occur in clay- 

 too*. slate. 



Atamiiitc. 6. Alum tlate. Trm rock is clay-slate impregnated 

 with carbon and sulphur. The sulphur is either com- 

 bined with the carbon, or united with iron, forming iron 

 pyrites. On exposure to the weather the sulphur be- 

 comes oxygenated, is converted into sulphuric acid, 

 which acts on the alumina of the slate, and thus forms 

 a sulphate of alumina. 



7. Drawing tlate, or black chalk. This also is clay- 

 slate, but more highly impregnated with carbon than 

 the alum-slate. When soft it is cut, and forms crayons 

 for sketching. 



8. Glance coal. This mineral occurs in beds subor- 

 dinate to clay-slate and greywacke, which sometimes 

 contain vegetable impressions. Of this, there are ex- 

 amples in Switzerland and in the kingdom of Saxony. 



9. Compact felspar. Beds of this mineral occur in 

 the transition clay-slate and grey-wacke of Dumfries- 

 shire, and along with rocks of the same description in 

 France, Italy, and Spain. 



10. Greenstone. This rock is met with in the tran- 

 sition ranges in the south of Scotland, and in similar 

 mountains on the continent of Europe. 



1 1 . Vegetable remaint* Transition clay-slate occa- 

 sionally contains vegetable impressions, particularly 

 those varieties which are associated with glance coal. 

 Animal remains are seldom met with in the clay-slate, 

 and still less frequently in the grey-wacke. These are 

 mndrrporitrs, Irilobilei, ammonites of a particular descrip- 

 tion , and hysterolilei, which are, in some degree, cha- 

 racteristic of this formation, and which appear to be 



4 



Drawing 

 or 

 i chalk 



OftM 



Celipar. 



Graca- 



Vegetable 



II. Trantition Limettont. 



llbergangs Kalkstein. Werner. 

 Transition Limertone. Jameton. 

 C'alcaire Intermediate. Daubuiuon. 



Transition 

 lime* tone. 



1. Character!. Thit limestone is in general more Characters. 

 compact than that met with in decided primitive dis- 

 tricts ; yet it is not always so, for it occasionally occurs, 



is coarse granular or highly crystallized. Its frac- 

 ture is, in general, splintery, usually combined with 

 ited, and is translucent on the edges. 

 It varies very much in colour. It is often black, 

 and frequently many colours occur together, these 

 forming what are termed variegated marble. Many 

 t. I'll. i- varieties are traversed by small cotemporaneous 

 veins of calcareous spar. Mos-t of the ornamental mar- 

 bles used in architecture belong to tin- munition class, 

 while the ttatuary marbles are of primitive, and the 

 coarser marbles ot secondary formation. 



2. Subordinate Bed* and Imbedded Mintralt. (l.)Sub*rdi- 

 I.tfdian Stone. It occurs abundantly in the bitu- naie b*d 

 luinous transition limestones in the north of France, lnd lm| *<*- 

 wherc it is disposed, either in irregular masses like flint ^ raine " 

 in chalk, or it forms small plates or tables, or it alter- -dj tn 

 nates in beds. The same disposition is met with in the n' on e. 

 transition limestones of the Pyrennees, Alps, &c. 



(2.) Mica occurs disseminated or in layers, and some- Mka. 

 times associated with talc or steatite. 



(3.) Quart*. It occurs disposed in veins, beds, or Quartz, 

 disseminated, and sometimes in the form of rock cry- 

 stal. 



(4.) Pyritet. This ore is generally denominated, Pyritei. 

 ISron-n Ircmttone. Occurs in veins and beds. g ro ' wn 



(<.) (ilance-coal. Some black transition limestones iron , lone . 

 are highly impregnated with carbonaceous matter, which Glance- 

 is sometimes accumulated in particular points, and thus coal. 

 forms imbedded masses of glance-coal, of which there 

 are examples in the north of France 



3. Petrifactiom. Fossil organic remains occur but Petrifac- 

 rarely extensively distributed in this rock, but aretiom. 

 sometimes abundantly accumulated in particular situa- 



" Petrified madrepores, millepores, ortboceratites, 



