448 SKETCH TOWARDS A 



founded with the true tertiary ones, is what I cannot 

 help ; when, in some of the cases, the causes are iden- 

 tical. I am still bound to separate these from the al- 

 luvia, at some point: if I have not chosen the best, it 

 will be but for others to alter the order : the theory 

 itself will not be affected. 



A lake receives alluvia, and forms strata, just as a 

 sea does ; and lakes have done this in antient times. 

 Such a lake also may have been fresh or salt ; or it 

 may have been salt at first, and subsequently freshened. 

 Partial elevations of land have occurred, demonstrably, 

 in times before our own : and, affecting these, they 

 are elevated lacustral deposits, salt, or fresh, or both. 

 Similar strata are, and therefore have been, produced 

 in aestuaries. Under the same circumstances, they 

 now form tertiary deposits ; while variations of cha- 

 racter with respect to the marine or terrestrial quali- 

 ties of the strata, may have arisen from terrestrial in- 

 undations, or from marine ones, or from vacillations in 

 the level of the land. And these seem to me all the 

 deposits which, in a just Theory of the Earth, should 

 occupy this place : because they are included within 

 certain antient revolutions : while it will be more con- 

 formable to an order tracing the progress of the globe, 

 to rank the deposits of lakes which have been drained 

 or filled, with the alluvia ; as we rank travertino. 



Thus, with the exception of one important subsi- 

 diary point, as yet inexplicable, and of some others, of 

 a subordinate nature, which I hope to explain, I have 

 terminated this sketch of the Theory of the Earth, 

 down to the period of its commencement under the 

 existing condition, with the reservation formerly made 

 respecting the volcanic islands. These minor matters 

 I shall here note, as briefly as possible, before I pro- 

 ceed to that which concerns our own days. 



On the theory of mineral veins, so important as to 



