XVI INTRODUCTION. 



for the most part primary, chiefly of granite, gneiss, and 

 slate, in all the higher tracts, and even in most of the 

 lower, in which latter there is a thick covering of diluvial 

 matters, consisting of the dehris of primary rocks. 

 Secondary strata, however, occupy considerable portions 

 of the lower tracts, especially in Forfarshire, and in the 

 northern parts of Aberdeenshire. Although not gene- 

 rally remarkable for fertility, and but scantily wooded, 

 it presents all the varieties of situation fitted for the in- 

 habitation of Molluscous Animals, which, however, are 

 much more numerous toward the sea. From Bervie to 

 a little north of Stonehaven the coast is rocky, and com- 

 posed of conglomerate and red sandstone ; from thence 

 to Aberdeen, of gneiss rocks, intersected by granite and 

 ignigenous veins. From Aberdeen to a little beyond the 

 Ythan, is a sandy beach margined by hillocks of drifted 

 sand. A rocky coast of gneiss succeeds, until we come 

 to the sandy Bay of Cruden ; from which to Peterhead 

 the rocks are chiefly of granite and gneiss. From thence 

 northward, and round the north-eastern extremity of the 

 district, to Troup Head, are sandy beaches, gneiss cliffs, 

 and latterly red sandstone and conglomerate. Grey- 

 wacke, sandstone, and primary rocks succeed. The sea, 

 generally shallow along the sandy coasts, and slowly 

 deepening eastward, has its bed partly rocky, partly of 

 sand, and sometimes of clay and other detritus. 



The currents being strong, and the coast exposed to 



