126 



IGNEOUS ROCKS OF 



TJpppr 

 Gallery. 



42. 



Dolwyddelan, Caernarvonshire. Ma 



Wall-case 46. 



P 



and 



"Wall-cases 

 lto6. 



75 N.E, Caradoc or Bala beds, showing joints 

 cleavage. The top, bottom, and sides are joints, and the 

 planes in which it is split are cleavage planes. 



This specimen forms part of a small block bounded by 

 joints, and suitable for splitting into pieces for the manu- 

 facture of small slates. It has been split into three pieces 

 but would have admitted of finer division. The value of 

 slaty rocks much depends on the small number of joints 



The fewer the joints the larger 

 are the slabs and slates that may be manufactured from the 

 blocks extracted from the quarries. 



and their arrangement. 



43 to 48. — Slates of various kinds. 43 and 



e from the slate quarries of Penrhvn and Llanhpri 



48 



the Cambrian rocks of Caernarvonshire ; 44 is from the 



North 



brokeshire 



Ffestiniog 



from the slate quarries in the Caradoc or Bala rocks of Dol- 

 wyddelan ; and 47 from the Devonian rocks of Tintagel, 

 Cornwall. 



49 to 53. — Various specimens of the darker cleaved 

 purple rocks of the Cambrian slaty region of Llanberis. 



IGNEOUS ROCKS. 



The contents of Wall-Cases 1 and 2* and the table 

 cases in recesses 4 and 6, are intended to illustrate the igneous 

 productions of tertiary and existing volcanos, and for com- 

 parison with similar products of more ancient date, collec- 

 tions of which are placed in Wall-Cases 4 to 7. 



Case 1 contains specimens of the volcanic rocks of Aden, 

 1 to 19 ; and of the volcanic rocks and products of eruption 

 of Hawaii, numbered from 20 to 42a. 43 to 121 are from 

 the {tertiary?) volcanic rocks of the mining district of 

 Schemnitz, in Hungary, from Croatia, and Transylvania. 





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