278 
HUGHES : INGLEBOROUGH. 
horizon from that {g of section, Fig. 4) characterised by the large 
specimens of the normal form of Producta gigantea which occurs 
further down stream (see page 276). The Upper or Black Marble 
Series occurs in thicker beds as we follow it down stream (c, 
Fig. 4), and, in consequence, instead of producing shallow wide 
ledges, gives rise to small gorges through which the water rushes 
in cascades, as is well seen in Plate XXXV., Fig. 1. 
This rock is on the same horizon and has much the same 
character as the Black Marble of Dent. In fact, where the 
quartzose sand of the overlying series is carried by the stream 
over the surface, these beds are beautifully polished and curious 
effects are produced owing to the interruption of the current 
here and there by small grains of harder material or protuberant 
fossils whose crystalline texture better resists the action of the 
sand and water. 
Flint occurs abundantly, especially in the higher beds, 
and a new fracture shows clearly the marginal portion of the 
segregations which is generally of a lighter colour than the 
black limestone in which it occurs, or the blacker flint within. 
These marginal portions probably represent the limestone ijn- 
perfectly replaced by silica, and to them the term " chert " 
would be more properly applied than to the black nucleus. 
It seems to me that a very important change sets in between 
the Lower Series of Black Limestones, with the large normal 
Producta gigantea, and the Upper Black Marble Series, with 
Producta latissima, Phil., and I would feel inclined if I were 
drawing a line in a new country to take the base of the Yoredale 
rocks between these two, viz., at the base of the shale, between 
the Upper or Black Marble and the Lower or Black Shaley 
Limestone Series. This, however, is not a matter of very great 
consequence, for I quite agree with what Salter used always 
to tell me, that the Yoredale rocks were but the splitting up 
of the top of the Mountain Limestone. If this be so, we may 
expect considerable variation in detail from place to place, 
though the general characteristic is that the top of the grey 
limestones of the Great Scar begin to show more and more 
carbonaceous and clayey material, and sometimes one, some- 
times another, Idnd of sediment predominates in different. 
