Chap. 12.] GEOLOGY, BOTA>rY, &c. 253 



silted up, and a growth of vegetation took place. Subsequently these 

 areas were again submerged, and became quiet during the formation 

 of the limestone. This was followed by a second period of rest, 

 silting up, and so on. It requires no great stretch of the imagination 

 to picture this alternation of conditions. To the north and west, on or 

 near the shore of the great north-west continent, there were numerous 

 active volcanoes, as evidenced by the extensive interbedded volcanic 

 rocks of this age in Scotland and Ireland. While these volcanoes 

 were in eruption the sea-bottom would probably undergo depression, 

 but so soon as the pressure was relieved the lava wovdd cease to flow 

 and a period of comparative rest ensue. Without doubt it may be 

 stated that after the deposition of the Mountain Limestone, there 

 was a general but interrupted shallowing of the carboniferous sea, and 

 that during this period the Yoredale Eocks were laid down. 



Fossils are of uncommon occurrence in the Yoredale Eocks, but a few 

 species common to the mountain limestone have been found. Long- 

 ridge Fell, the Bleasdale Fells, Pendle, and Ingleborough, afford 

 examples of the gi-its of the series, while the shales may be seen in 

 various outcrops over a large tract immediately to the north-west of 

 Longridge. 



THE Mn-LSTOITE GRIT. 



This deposit is mainly estuarine, and consists chiefly of a coarse 

 gritty sandstone, with occasional thin seams of shale, coal, and, in some 

 areas, ironstone. 



Possibly there was a slight upheaval of the previous submarine 

 area, so that the shallower portions became land, enclosing i mm ense 

 inland seas. Into these seas numerous large rivers rapidly deposited 

 their burden of detritus torn from the older land areas. There is 

 evidence both in the MiUstone Grits and the Yoredale Grits that they 

 resulted from the rapid denudation of granitic rocks. Mr. A. J. Jukes 

 Browne has pointed out that the measure of the detritus must be a 

 measure of the land from which it was worn. The Yoredale Eocks and 

 the Millstone Grits taken together give a total thickness of nearly ten 

 thousand feet. The continent from which such a mass of detritus was 

 worn must necessarily have been a large and probably mountainous 



