1890.] NATURAL SCIKNCKiS OF PHILADELPHIA. 309 



pos, Medio, Cabeza, Anegada de Afuera, etc.) are considerably 

 larger than those of the first, the Chopos reef measuring nearly three 

 miles in length (N. W. — S. E.) Owing to the limited time at our 

 command, we were unable to visit these lower reefs, and from per- 

 sonal observation, therefore, I am unable to say in how far they are 

 purely of coral structure. But from the statements of those who 

 are acquainted with the waters, I gather that they are largely identi- 

 cal in formation with the islets lying off Vera Cruz, and the.ir general 

 l)Osition and direction lead me to infer that tliey have a common 

 origin. On tlie hydrographic charts all the dangerous banks are 

 marked as reefs, but this should not be taken to necessarily mean 

 " coral reefs." Thus, the reefs off Punta Gorda, lying 3-4 miles to 

 the northwest of Vera Cruz, I found to be serpuloid and not coral, 

 and the same is true, in great measure, of the Hornos reef, which lies 

 about a mile to the southeast of the city. It should also be stated 

 that the " coral sand" of hydrographic charts is not necessarily a sand 

 of triturated coral, but may be of other limestone formation. 



An examination of the accompanying maps (Pi's. VI and VII) 

 shows that the main axis of nearly all the islands is directed in a N. W. 

 — S. E. line, or in a direction parallel with the coast, a condition doubt- 

 less due to the interaction of prevalent winds and local currents, the 

 latter of which shape the position of the detrital material. This is 

 well shown by the heaps of shingle, coral-fragments, shells, etc., which 

 have accumulated on the southeastern faces of some of the banks — 

 the general lee-side of both wind and current — either as the result 

 of long-continued action or of a single storm. Such deposits are 

 especially well developed on Blanquilla and the Isla Verde ; the isl- 

 and portion of the Sacrificios reef is likewise situated in the south- 

 eastern half of the circle of coral by which it is surrounded. Much 

 the same contours that we see in the banks to-day already existed a 

 century ago, as may be learned from the magnificent chart prepared 

 by Ponzoni, in 1807, for the use of the Spanish Hydrographic 

 Service.! This shows that the dry mass of Blanquilla already then 



1 This extremely rare map of the harbor of Vera Cruz, of which, I am in- 

 formed, only two copies exist to-day in the whole of Mexico, was published in 1816. 

 It is by far the most detailed of all the maps illustrating this portion of the Gulf 

 waters, and it is especially interesting in connection with the study of coral reefs, as it 

 carries back the authentic history approximately one hundred years. For the use 

 of this map, and the permission to make a copy of it, I am [indebted to Captain 

 Powell, of Vera Cruz, Chief of Construction of Docks of the Mexican Railway. 

 To the same gentleman, and to Messrs. Hall and Santiago Shirley, representing 

 also the Mexican Railway, I am further indebted for many facilities offered for 

 the exploration of the reefs and the accomplishment of our mission generally. 



