ECONOMIC GEOLOGY. 281 



CORRESPONDENCE. 



PoKT OF Spain, Trinidad, April, 1857. 



My Dear Sir : In my last letter I gave you some idea of the geo- 

 logical structure of this singular island; and also the meteorological fact 

 that hurricanes are never known here, although they have occurred at 

 Tobago, 30 miles east of us. By the accompanying registers you will 

 he enabled to see the minute variations of our mountain barometer, 

 made by Messrs. Troughton & Simms, London. Our aneroids have 

 done good service, particularly that invented by Bou.rdon & Richards. 

 Our boiling water experiments have not been so satisfactory ; I am 

 induced to attribute this to the great humidity of our atmosphere. 



I enclose you also a copy of our report, as it may interest you. 

 Our future operations will be over a more interesting country. This 

 report you must not regard as a scientific document, but one writ- 

 ten to meet the capacities of the inhabitants of this island. As 

 our future labors are published, if you take any interest in them, I 

 shall have much pleasure in sending them to you. They will em- 

 brace our survey of that interesting object, the " Pitch Lake." The 

 difficulty of transit over the low lands during the rainy seasons pre- 

 vents our carrying on the survey with that system of regularity so 

 desirable. Moreover, we have to limit our researches to very superfi- 

 cial examinations, as they are intended for mineralogical rather than 

 for geological purposes. Various local causes operate against us, partic- 

 ularly the density of the vegetation, which surpasses that of every 

 other country I have visited except the portion of New Granada near 

 the Isthmus of Panama. 



I remain, my dear sir, yours, very sincerely, 



JAMES G. SAWKINS. 



Professor Joseph Henry. 



REPORT OF PROGRESS, FROM AUGUST 25, 1856, TO FEB- 

 RUARY 24, 1857, OF THE SURVEY OF THE ECONOMIC 

 GEOLOGY OF TRINIDAD. 



BY G. P. WALL AND JAS. SAWKINS. 



An examination of the Economic Geology of a country necessarily 

 includes several kinds of observations and investigations, viz : 



Those appertaining to a Geological Survey proper, in which the 

 nature of the different rocks and strata are ascertained, their mutual 

 relations to one another defined, and the various disturbing causes 



