GEOLOGICAL .FEATURES 



OF 



MARLBOROUGH. 



IN the study of the surface or crust of the earth, there is 

 abundant evidence to prove that it has always been subject 

 to the same denuding and restoring agencies as those which 

 are at present at work on it. 



These agencies are of two kinds, internal and external. 

 The internal or igneous agencies, such as earthquakes 

 and volcanoes, elevate or depress the earth's crust, either 

 at intervals or by slow degrees, and throw out matter from 

 its interior ; while the external or aqueous agencies, such 

 as rain, rivers, currents, icebergs, &c., are incessantly 

 stripping off the soil from one part of the earth's surface 

 and depositing it in another. 



In considering the geology of the neighbourhood of Marl- 

 borough we are only indirectly concerned with the internal 

 agencies, as the surface for many miles round, and to a 

 great depth, has been entirely formed by external agencies. 

 Before entering into detail, it will be well to consider the 

 different forms of external agency. Every stream or river 

 is constantly rolling and carrying along with it portions 

 of the clay or sand forming its bed, flowing faster and 

 carrying away more soil as its course is steeper, or as other 

 streams unite with it. The loose material swept down by 

 rivers, which is much increased by heavy rains and floods, 



a 5 



