122 



Scientific Geology. 



spread uniformly over the whole globe, the ocean would flow over 

 the whole of it, even with considerable depth of waters. 



In precipitous ridges, particularly of trap formation, frost com- 

 mences the work of degradation. Water, penetrating the fissures of 

 these rocks, expands by freezing and forces them slightly asunder. 

 This makes room for a larger portion of water the succeeding win- 

 ter ; and thus the process goes on until the columnar masses of rock 

 are urged downward by the force of gravity and powerful rains. 

 This is the origin of those extensive slopes of broken fragments, or 

 debris of rocks, which arrest the attention on the mural faces of the 

 greenstone ridges in the Connecticut valley. Generally these frag- 

 ments rise only about one half or two thirds the height of the ridge ; 

 though sometimes they continue to the very summit : the process of 

 degradation from this cause having come to an end. 



Instances of this kind have been regarded by geologists as a 

 kind of natural chronometer, demonstrating the recent origin of the 

 present state of the globe. No observations, however, have been 

 made on the progress of this leveling process accurate enough to 

 compare it with historical records. 



When the three causes of degradation above mentioned combine 

 their maximum energy on the sides of steep Alpine summits, they 

 produce the well known and sometimes terrific phenomenon of land 

 slips. Though examples of these on a limited scale are very com- 

 mon in Massachusetts, yet the only one worthy the particular at- 

 tention of geologists, is on the southwest side of Saddle Mountain, 

 at the place called the Hopper. But this has been particularly no- 

 ticed in the second part of my Report. 



Bog Ore. 



In the western part of Worcester County, and over a large extent 

 of territory, the process by which this ore is produced and deposited, 

 is so manifest that it deserves description. The gneiss rock there, 

 abounds with the sulphuret of iron. This is continually undergoing 

 a decomposition by the action of heat, air, and moisture ; and be- 

 comes changed, first into an oxide, and then, some of it into a sulphate. 

 The oxide usually imbibes more or less of carbonic acid from the 

 atmosphere, and is changed into a carbonate ; which is soluble in wa- 

 ter. Or this oxide, being washed from the rocks by rain into cavities, 



