267 



Chalk, oe White Limestone. 



In examining a country geologically, where there are varieties of 

 rock, it is a great advantage, as a help towards determining the rela- 

 tions of the different parts, to find one band or bed remarkable for 

 some physical difference from those which accompany it, and follow that 

 out through the whole district, so far as it can be done. For this pur- 

 pose, among the rocks of Antrim the Chalk affords an eligible index of 

 this kind. There are two circumstances connected with it favourable 

 for this purpose. The first is its general outcrop, about midway from 

 the bottom to the top of the steep escarpment which occurs along near 

 the east coast of the county, all the way from Lisburn to Cushendall, a 

 distance of 40 miles : the second is its very white colour, which makes 

 a strong contrast with the black overhanging precipices of trap, by 

 which it can be recognised for several miles by land or by sea within the 

 range of vision. 



The chalk of Antrim, as a whole, is indurated, and much harder 

 than the chalk of England. It is called in the country white lime- 

 stone — a name even more familiar to me than chalk. 



ATy observation leads me to the conclusion that the chalk was laid 

 down upon an uneven bed, because, although we find it in many places 

 of nearly equal thickness, and only a slight inclination from the hori- 

 zontal, yet in other places it is very thin, which argues a shallower sea 

 in those places, and of course a higher sea bottom, while in other loca- 

 lities there is no chalk at all, showing either that those parts were over 

 water at the time of the deposition of the mass, or that it was first de- 

 posited over the whole area, and afterwards those bare parts elevated 

 to the surface of the ocean, or near it, and then denuded. 



Here I speak only of the thick and thin parts of the chalk and 

 where there is none. Those three cases occur at nearly the same level, 

 at stations No. o, 22, and 23, on the map (see PI. XXII.), and the fol- 

 lowing table. This table has been made for the purpose of explaining 

 the outcrop of the chalk more clearly. Other localities there are where 

 the chalk is 200 feet thick at sea level, as it is at station Xo. ST. It is 

 only three feet thick at Xo. 28, which is 400 feet higher, and at Xo. 5, 

 where it is 130 feet, at 680 feet above the sea. 



There appears to be no rule by which we can expect it to be thick 

 or thin, high or low in any locality. Subsequent dislocation has pro- 

 bably acted upon it in such a way as to baffle any attempt at specu- 

 lating in this way. 



The surface of the carboniferous rocks in the bottom of the ocean, 

 previously to the deposition of the new red sandstone, appears to have 

 been uneven in the county of Antrim. Otherwise the accumulation of 

 this rock at Belfast, which exceeds 2000 feet, as already stated, and 

 at Duncrue and Red Hall, near Carrickfergus, where it has been 

 bored to nearly an equal depth below the overlying chalk, would be so 

 much greater than it is at Cushendall, where the very base of it is 

 visible, and where, a mile or two west of this place, it thins out to 

 nothing. The hollows having been filled up with the red sandy deposit, 

 the upper surface has been brought more nearly to an even plane than 



