May 25, 1857.] PHYSICAL GEOGRAPHY. 417 



stream was unquestionably occupied, at a very modem date, by 

 a great internal sea, the desiccated shells of which, now lying on 

 the steppes,* are of the same species as those still living in the 

 Caspian. 



In these dried-up bottoms of a vaster Caspian, or what I termed 

 " Aralo-Caspian," * the erratic blocks of the north are no longer to 

 be seen, and we are in a region where the right bank of the Yolga 

 has been fashioned into cliffs by the agency of winds and currents 

 proceeding from a point of the compass very different indeed from 

 that whence the winds and waves proceeded, when the cliffs ranging 

 from Nijny to Kazan were formed. 



In thus cautiously reasoning from data which are absolutely in 

 our possession, and by extending the application of existing causes, 

 we may be capable of determining the direction of the prevailing 

 winds in different epochs of the earth's formation, and even in very 

 remote geological periods ; for many of the escarpments of ancient 

 stratified rocks have doubtless liad their prevalent direction of 

 cliffs formed by the breakers and atmospheric agency of by-gone 

 periods, f 



Again, as we know that the ripples on the surface of the sands of 

 the present shores indicate the direction of the waves, so when a 

 sufficient number of observations shall have been made by Mr. Sorby 

 and others if upon the ripple-marks which have been preserved in 

 the successive surfaces of stone, we shall be enabled to infer the 

 direction in which the prevailing winds blew during each former 

 geological period ! 



But I am now, perhaps, realizing too demonstratively for all my 

 hearers, the truth of the incontrovertible axiom, that physical geo- 

 graphy and geology are inseparable scientific twins. 



* See • Russia in Europe and the Ural Mountains,' p. 299, and the Geological 

 Map, on -which are noted the two points here contrasted, viz. — the southern 

 range of the northern erratic blocks and the western boundary of the Aralo- 

 Caspian Deposit. 



+ See the account of the formation of the * Straits of Malvern,' in Murchison's 

 'Silurian System,' p. 530; and consult Professor Ramsaj's writings on this point 

 in his ' Memoir on the Denudation of England and Wales,' * Memoirs of the 

 Geological Survey of England and Wales,' vol. i. p. 333. 



X See ' Edinburgh Phil. Mag.,' New Series, vol. iii., p. 112, 1856. Mr. Sorby 

 has particularly distinguished himself by his numerous observations on this 

 subject, and has also explained his views by ingeniously contrived instruments 

 of his own invention. 



