NOTES ON THE GEOLOGY OF THE BAHAMAS 39 



its roots proceeded downward a few inches through the solid rock. At 

 the surface the trunk expanded so as to form a projecting mass that 

 rested on the rock. The bush was only about eighteen inches in height 

 and evidently stunted by lack of nourishment. I then pulled up a 

 number of shrubs in the vicinity, and found that their roots ran under 

 the hard crust that formed the surface of the rock. Further observa- 

 tion showed that in many places the crust had been lifted and broken by 

 the growth of the roots, and then trees were noticed with the base of 

 their trunks surrounded by slabs of rocks that leaned against them. 

 Finally, when the rock becomes eroded, the roots of trees penetrate the 

 holes and crevices, and by their growth crack off large fragments that 

 subsequent erosion forms into boulders. And these boulders are found 

 most abundant where the trees are the largest, and hence where the 

 action of their roots is most vigorous. Hence we may infer that these 

 blocks are formed by erosion and the growth of the roots of the shrubs 

 and trees. 



While at Nassau I noticed on the shore in some places — as near 

 Dix Point — vertical masses of rock that ran in an irregularly curving 

 or straight line on the surface. The projections were sometimes about 

 one foot high and two to four inches in width, and containing on their 

 upper edge a number of holes, about half an inch in diameter, that were 

 often shallow and sometimes mere pits, but quite regularly distributed 

 in a single line. These little ridges could sometimes be traced for a 

 distance of fifteen or twenty feet, and had evidently been left standing 

 while the surrounding rock had been eroded. For a long time I was 

 unable to explain their formation. Sometimes the ridges intersected 

 and formed small or large triangles, as the case might be. When I 

 visited the south side of New Providence, I saw growing in the water, 

 in the calcareous mud that there forms the bottom, the black mangrove 

 or salt bush {Avicennia nitida), and radiating from it, projecting about 

 five or six inches above the ground, were small vertical shoots about 

 three or four inches apart and looking very much like the teeth of a 

 very long wooden rake. These shoots, I afterward found, come from 

 the long sucker-like roots of the Avicennia and also of the button- 

 wood (Conocarpus erectus). Later, on the west side of Andros, I found 

 these plants growing near the water and also higher up on the beach, 

 which here was a very fine calcareous deposit. This deposit had been 

 slightly raised by the growth of the shoots, and higher up on the shore, 



