28 



Mr. W. J. Hamilton and Mr. H. E. Strickland, on the Geology of 



The outbursts of volcanic matter appear to be referable to three great periods.' 

 How long may have been their duration, or how long the intervals of repose be- 

 tween each, is buried in the tomb of time. All that we can now assert, is, that 

 long intervals must have passed between each period of eruption ; and that the 

 latest eruption occurred antecedently to the commencement of traditional or au- 

 thentic history. 



The- oldest series of eruptions took place at a time, when the bed of the lake pre- 

 sented a nearly level and unbroken surface, and before the first commencement 

 of the excavation of the present valleys ; for the basaltic rocks of that period in- 

 variably form the capping of the vast horizontal plateaux of tertiary lacustrine lime- 

 stone. These eruptions could not have occurred long before the drainage of the 

 lake ; for if they had, we should find sedimentary deposits overlying the volcanic 

 rocks, which is not the case ; nor could they have occurred long after that event, 

 for the valleys of denudation were not then formed. The eruptions of the second 

 period were subsequent to the drainage of the lake, and to the excavation of deep 

 valleys in the lacustrine deposits. Those of the third period are still more recent, 

 and are distinguished by their entire identity of character with volcanos now in 

 action. We will describe the localities and phsenomena of these three classes of 

 volcanic rocks in the order of their antiquity. 



Section of columnar Basalt (for description, see next page). 



