DISCUSSION OF TEERESTRIAL SUBMEEGENCE. 21 



A consideration of the continental elevation and its effect on the erosion of the 

 valleys ; the differential depression of the land which followed the elevation ; the 

 different epochs of variable movement ; the biologic evidence noted, and other 

 questions are portions of the study an advance notice of which is here recorded- 

 To Messrs Jukes-Brown and Harrison, whose discoveries of oceanic deposits in 

 Barbados were accepted by themselves and others as evidence of an upward move- 

 ment of 2,000 fathoms and more, the author owes much encouragement in ventur- 

 ing, without precedence in this country, to bring before this Society evidence of 

 epeirogenic movements (apart from orogenio in very recent geologic times, amount- 

 ing to two and a half miles of vertical subsidence of great land areas, thus proving 

 the mobility of the earth's crust, although no explanation can be suggested at 

 present to account for such stupendous movements. 



Remarks upon this paper were made as follows : 



W. P. Blake : 



The extremely interesting paper of Mr. Spencer gives me an opportunity to 

 mention some unpubUshed observations which are pertinent to the subject. In 

 following the footsteps of Columbus in the island of Santo Domingo, I crossed the 

 line of what appeared to be a remnant of an ancient marginal corraUine limestone 

 reef at an altitude of about 2,000 feet above the sea. There was no opportunity 

 for extended examination, but it greatly resembled the corraUine limestone terrace 

 of post-Pliocene or modern age which borders this and some of the other West 

 India islands at the sea-level or a little above it. The occurrence was so startling 

 in its evidence of a recent elevation of the island as to cause me to refrain from 

 accepting its full significance; but in the light of further evidence I am now in- 

 clined to accept it as proof of a modern uplift and, of course, a former depression, 

 thus showing an oscillatory movement. 



The marginal limestone, as is well known, is but little raised above the sea-level, 

 and has long been exposed to aerial decomposition, by which it has become covered 

 with a thick red soil. 



W J McGee : 



Both the author and the Society are to be congratulated on a paper of this char- 

 acter, dealing as it does with the outlying phenomena of our continent, by which 

 alone its relations to other lands may be traced. It has long been recognized that 

 the great system of islands lying off the Florida coast sinuilate the peaks and 

 crests of a mountainous land, and it has been inferred by different geologists that 

 the apparent condition is the real one, and that these islands are the monuments 

 of a drowned continent. This inference is greatl}'^ strengthened by Doctor Spen- 

 cer's recognition of rain-fashioned slopes and river-carved valleys in the deeps 

 between the island-tipped mountains. Hitherto the profound crustal movement 

 indicated by these phenomena has not been correlated with the lesser crustal 

 movements of the North American mainland, and Doctor Spencer's provisional 

 correlations are accordingly of exceptional interest and value. 



While, as the author has shown, the general configuration of the island region 

 suggests geologic youth, there appears to be some reasons for questioning whether 

 the profounder movements were not considerably more remote. Inspection of the 

 relief map giving submarine configuration shows that the Bahamas and neighbor- 

 ing islands are bordered by submarine terraces, or, perhaps more properly, flanked 



