On the Absence of any Traces of the fall of Aerolites. 337 



The theorem here considered is usually treated by the 

 " Calculus of Operations," a method which has deservedly 

 received much attention of late. The employment of this 

 refined principle of investigation requires, however, more than 

 ordinary caution and circumspection : among other things it 

 must be observed, that the theorems to which it leads cannot 

 be generally true when they assume the form of series, whose 

 character is such that, when the symbols of operation are re- 

 placed by those of quantity, divergency takes place. The 

 present short paper illustrates this : the calculus of operations 

 gives the theorem here discussed under the form (A.), and 

 makes no provision for the important correction (B.). 

 Belfast, October 9, 1848. 



LI. On the absence of any Traces of the fall of Aerolites and 

 of Glacial Action in the Strata formed before the last great 

 modification of the Earth's surface. By Lieut.- Colonel 

 Portlock, R.E., F.R.S. %c*. 



IN no science have the results of a close adherence to the 

 sound principles of inductive reasoning been more striking 

 than in geology during the last fifty years. So long as the 

 mind was allowed to ramble unrestrained in the field of spe- 

 culation, hypothesis supplied the place of facts ; and the vision 

 occasionally obtained of truth was like that of a dreamer, so 

 confused and distorted as no longer to resemble any distinct 

 reality; but when geologists abandoned this pursuit of an airy 

 image, and began to search cautiously and laboriously for 

 facts as the only sure basis on which any system of human 

 knowledge could be founded, a new aera commenced, and 

 a science has now been established which in beauty, interest 

 and utility, is second to none. To this great end the work 

 of Mr. Lyell, Principles of Geology, most powerfully con- 

 tributed, by its advocacy in the strongest and most uncom- 

 promising manner, of the advantage of seeking only in exist- 

 ing causes (that is, in forces still in action) an explanation of 

 past as well as of present phaenomena. It is thus that not 

 merely the true structure and the mode of existence of the 

 strangest forms of extinct auimals have been elucidated by 

 a reference to the laws of anatomical structure and of organic 

 functions, as exhibited in the types still existing, but the 

 ripple-mark on the sea-shore or on the sandy bottom, the 

 scar of the pattering rain-drop, and the action of the sea 

 wave on the rocky cliff, have been carried back by various 

 observers to the remotest epochs, so that the mind of the geo- 



* Being the substance of remarks made in the Geological Section of the 

 British Association at Swansea. 



