12 Messrs. W.Phillips and Kent on [Jan. 



same direction and dip as the more perfect slates of the quarries ; 

 but although they occur in layers varying from one-eighth of an 

 inch to two inches in thickness, these layers have not the same 

 direction as that of the cleavage ; but this we shall presently 

 describe. 



The preceding may be said to constitute the principal varie- 

 ties of the imbedding substance or paste in what may be termed 

 its purer state ; in which, however, it is sometimes rendered 

 irregularly slaty, by the intervention of layers of chlorite ; the 

 substance then, as well as in some of the purer varieties, some- 

 what resembles potstone- 



The quartz which forms so important a feature of the slates, 

 and which doubtless is more finely disseminated throughout the 

 harder varieties of the rock in the state of a mechanical mixture, 

 occurs in two instances at least under very different circum- 

 stances ; in one as a bed or vein of considerable dimension and 

 extent, of granular quartz, having the aspect of quartz rock, and 

 consisting apparently of grains of pure and nearly transparent 

 quartz closely and strongly aggregated without any intervening 

 cement ; this bed or vein occurs in Grooby Park, and appeared 

 to us to run in the direction of the slaty cleavage observable in 

 the quarries and elsewhere, and with the same dip. Slates were 

 observable on each side, though not in close contact with it. 

 In the other instance, compact quartz of a green colour, and 

 which but for the translucency of the edges might be mistaken 

 for green jasper, exists in layers in a large mass in a field behind 

 the last house of Newton Linford, which is most distant from 

 Grooby. It lies immediately between layers of the granular 

 looking varieties of the coarse slates before described, and with- 

 out any precise line of demarcation. But this rock will be more 

 fully described in adverting to the subject of the stratification of 

 the rocks of this district. 



Hitherto we have not mentioned these rocks in their more 

 ordinary state of porphyries ; the aspect of these differs greatly 

 in different places ; but whatsoever their nature may be, they 

 are usually found connected with those we have already described, 

 and generally in layers alternating with them without any regu- 

 lar order of succession. Sometimes, however, they appear to 

 form almost exclusively the rocky eminences, already described 

 as receiving each its own individual name, and especially those 

 of the western part of the forest, near to Thrinkston. 



The imbedded substances of the porphyritic rocks are various; 

 these consist of transparent crystals of quartz, which generally 

 are small, or of translucent crystals of felspar, often very minute, 

 and which frequently are macles, or hemitrope crystals, and also 

 of cleavelandite ; these substances occur either separately 

 (Broad Hill, Bardon Hill, &c.) or together in the same mass 

 (High Swanmore, High Sharpless, &c.) and they are rarely 

 abundant. Not unfrequently, however, instead of regular 



