S30 Correspondence — Mr. Warren Upham. 



the contours of the hills and valleys, the author considers that it 

 is clear that the main physical features of this portion of N.W. 

 Middlesex were moulded at a very early stage in the Glacial 

 period, and before the so-called Middle sands and gravels and over- 

 lying Upper Boulder-clay with Northern erratics, were deposited. 

 He believes that at this time there could have been no barrier of 

 any importance to prevent these deposits from extending into the 

 Thames Valley, and that the evidence clearly points to the conclusion 

 that the implement-bearing deposits on the higher horizons in the 

 Thames Valley should be classed as of contemporaneous age with 

 the undoubted glacial deposits at Hendon, Finchley, and on the 

 slopes of the Brent Valley, which they so closely resemble. The 

 author is therefore satisfied that man lived in the neighbourhood 

 of the Thames Valley in the q&xXj part of the Glacial period ; 

 probably, he thinks, in pre-Glacial times. 



COiaieiBSIPOItiTIDJEIsrOIB. 



CORRELATION OF QUATERNARY CHANGES OF LEVELS IN NORTH 

 AMERICA AND THE CARIBBEAN REGION. 

 Sir, — Referring to Mr. Jukes-Browne's letter in this Magazine 

 for March (p. 143), and to his and my preceding articles and corre- 

 spondence therein cited, I have to reply that the questions which he 

 asks are manifestly very difficult ; but certain points may be noted 

 which partially answer them. If the end of the Tertiary era and 

 beginning of the Quaternary were characterized by elevation of the 

 greater part of the North American continent to such altitude as to 

 give a much cooler climate, and by a contemporaneous depression of 

 the West Indies and the Isthmus of Panama, allowing a large part 

 of the equatorial Atlantic current to pass into the Pacific Ocean, the 

 result of these combined conditions being the accumulation of the 

 ice-sheets of North America and Europe, we should expect to find 

 west of the Gulf of Mexico, as Mr. Jukes-Browne suggests, evidences 

 of recent changes of levels. The subsequent depression of the con- 

 tinent north of the Gulf and the uplift of the West Indies and the 

 Panama region, bringing the present relative heights of land and 

 sea, may have produced the east to west line of fissuring and faulting 

 which crosses Mexico near 19° north latitude, shown by the very 

 remarkable series of volcanoes of Tuxtla, Orizaba, Popocatepetl, 

 Ixtaccihuatl, Toluca, Jorullo, and Colima. Farther west, this line 

 of disturbance in the earth's crust probably extends to the volcanic 

 Eevillagigedo Islands. Eastward, after crossing the base of Yucatan, 

 it appears to be represented by the Great and Little Cayman Islands, 

 and by the Sierra Maestra on the south shore of eastern Cuba, with 

 the contiguous " Bartlett Deep," a very profound narrow trough of 

 the Caribbean Sea, reaching from Honduras Bay to the Windward 

 Passage ; and thence the same orographic belt is continued by Santo 

 Domingo, Porto Rico, and the Windward Islands, with the very 

 deep oceanic troughs north, south, and south-east of Porto Rico 

 (bathymetric map of the Caribbean region, A, Agassiz, Three 

 Cruises of the " Blake," vol. i. fig. 57). 



