234 CARNEGIE INSTITUTION OF WASHINGTON. 



Borings made by Mr. Mills, for the Florida East Coast Railway, between the 

 lines of Fourth and Tenth streets extended, to a distance of 1,700 feet off the 

 point of the Terminal dock, showed a rock floor at a depth of 10 feet below 

 low tide, but narrow, well-like holes over 35 feet deep and filled with mud and 

 sand were found in several places, indicating the presence of solution wells in 

 the oolite. In a group of several holes, the drill after penetrating 4 feet of 

 rock, at a depth of 14 feet below the water-level, dropped 14 feet, and no 

 washings were obtained from this interval. This is evidently a cave having 

 a roof of 14 feet and a floor 28 feet below sea-level. 



The freshness of the elevated scarp in South Miami supports the inference 

 that the last movement was one of elevation of 10 feet. If this inference is 

 correct, the evidence indicates that the surface about Miami, after being below 

 sea-level, was elevated by stages to a position 55 feet above sea-level, then was 

 depressed 46 feet, and finally elevated at least 10 feet, resulting in the land 

 surface still standing 36 feet lower than previously. 



The scarp in South Miami was traced to the Punch Bowl, thence through 

 Cocoanut Grove and southwestward to Cutler. Southward from the Punch 

 Bowl the surface of the lowest terrace is occupied by a marsh or swanip 

 extending along the bay front. Southwest of Cutler the scarp decreases in 

 height until at 3.3 miles southwest it is not marked; 3.75 miles south of Cutler, 

 swamp appears on the oolite surface, and the scarp has disappeared though 

 the pine forest characteristic of the upland persists. The elevation in Miami 

 has been accompanied by local warping, but this was not sufficient in amount 

 to offset the antecedent dominant submergence. 



Shaw critically examined the submerged scarps around Tortugas with the 

 object of testing Vaughan's suggestion that they were due to marginal marine 

 cutting. These submerged terraces, and particularly the remarkable front of 

 a terrace Ijang 15 or 20 feet below sea-level, are the most important of the lines 

 of physiographic evidence of subsidence in the vicinity of the Tortugas. This 

 front is an undercut cliff and apparently differs in no essential respect from the 

 cliffs which are being formed to-day along the shore of southern Florida, where 

 the sea is cutting into a low-lying limestone area of land. 



The only other tenable hypotheses for the origin of these submerged under- 

 cut cliffs are, first, that they are due to some remarkable combination of rock 

 hardness and current work; second, that thej' have been produced by the 

 growiih of corals. The first hypothesis seems to be inapplicable (a) because the 

 submerged cliffs, though in detail intricately irregular, display in a broad way 

 the sweeping curves characteristic of shore-lines, are perpendicular to the 

 general slope of the bottom, and the cliffs are accordant in depth; (b) because 

 the sea-floor does not display the mushroom-like forms of all manner of size 

 and distribution which would result from the erosion of two hard layers 

 separated by a soft one; (c) Shaw, using diving apparatus, examined the rock 

 in the reentrant and found it as resistant apparently as that above and below. 

 The objection to the second hypothesis is that although coral skeletons are 

 very abundant on the edge of the overhanging cliff, as might be expected, 

 because such a cliff forms a very favorable place for corals to grow, other kinds 

 of rock are also present and the resulting feature does not have the form of a 

 coral stack, though coral stacks are present in the region. How extensive the 

 non-coral rock is can not now be stated, because of the difficulty in obtaining 

 samples. Some samples of Texas rock and of the rock at the edge of the cliff 

 0.5 mile north of the Tortugas laboratory were, however, obtained. 



Where the scarp is not covered with sand the surface just back of the top of 

 the cliff shows the deep pitting with many little channels and bores charac- 

 teristic of limestone cliffs along the present coast. The rock is stratified and 

 shows everywhere both large and small pits of extremely irregular form, such 



