Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/52

of the writer who has investigated the subject most carefully, we will name dysgeogenous and eugeogenous, names derived from the comparative readiness with which they produce detritus, and yield to disintegrating influences. In the North of England the first is typically represented by the limestones of various ages, and the other by the sandstones and shales from the Kimmeridge clay down to the Old Red. We see the two opposite types of scenery contrasted better in Yorkshire than in Northumberland and Durham. In the North Riding the eastern third is occupied by two parallel ranges of hills, running from west to east, for a length of 30 miles, from the central vale to the coast, one formed from the arenaceous rocks of the lower oolite formation, and the other from the calcareous upper oolite, which show the characteristic features of the two types excellently. The flat table lands of the limestone hills contrast with the irregular undulations of the sandstone hills: the steep precipitous calcareous scars, not less so with the irregular "edges" of freestone and gritstone. The sandstone hills are intersected by branching rivulets, which flow from their upper levels gradually down their slopes into the low country. The limestone hills have neither streams nor natural pools upon their surfaces, but the glens slope suddenly, and the water sinks through the calcareous beds to gush out in large volume when it reaches some less permeable stratum. The sandstone dales are open and irregular, with gradual slopes and undulated embankments; the limestone dales are steep and narrow, with sudden slopes and enbankments rising up like a wall upon each side to shut them in. In Northumberland and Durham we have only a limited and not very characteristic development of limestone hill and dale. The great series of beds deposited during the Mountain Limestone period, so characteristically calcareous in Derbyshire and the West Riding of Yorkshire, gradually loses this character as it passes northward, and as Mr. Tate has already explained, has changed completely by the time that it reaches Northumberland. Unless this state of things is kept in view, any one looking at our geological map may easily get a very wrong idea into their mind about the physical geography of our field of study. We have an instance of a