188 Description of the Patent reflecting Semicircle. 



its " Sovffriere." This '13 particularly the case with Nevis, 

 St. Kilt's, Guadaloupe, Dominica, Martinico, St. Lucia, 

 and St. Vincent's. Some islands have several such places, 

 analogous I presume to this of Montserrat; but in others, 

 as Guadaloupe, St. Lucia, and St. Vincents, there are de- 

 cided and well characterized volcanos, which are occa- 

 sionally aciive, and throw out ashes, scoriae and lava with 

 flame. The volcano of St. Vincent's is represented by Dr. 

 Anderson, and others who have visited it, as extremely large 

 and magnificent, and would bear a comparison with some 

 of those of Europe. These circumstances appear to have 

 been entirely overlooked by geologists in their speculations 

 concerning the origin and formation of these islands. It 

 has indeed occurred to most persons, on surveying the re- 

 gular chain of islands extending from the southern Cape of 

 Florida to the mouths of the Orinoco, as exhibited on the 

 map, to conclude that it originally formed part of the 

 American continent, and that the encroachments of the sea 

 have left onlv the higher parts of the land, as insular points 

 above its present level. But this hypothesis, however sim- 

 ple and apparently satisfactory in itself, will be found to 

 accord very partially with the geological structure of the 

 different islands. Many of them are made up entirely of 

 vast accretions of marine organized substances ; and others 

 evidently owe their origin to a volcanic agency, which is 

 either in some degree apparent at the present time, or else 

 niav be readily traced by vestiges comparatively recent. 

 There is every reason to believe, however, that some of the 

 islands are really of contemporaneous formation with the 

 adjacent parts of the continent, from which they have been 

 disjoined by the incursions of the sea, or by convulsions of 

 nature, and it is probably in those islands which contain 

 primitive rocks, that we are chiefly to look for a confirma- 

 tion of this supposition. 



XXXVI. Description of the Talent reflecting Semicircle^ 

 invented by Sir Howard Douglas, Bart. Communi- 

 cated by Mr. Carv. 



X HE objects of the reflecting circle or semicircle are to com- 

 bine the measuring principle with a circular or semicircular 

 protractor, in such a manner that, in measuring any angle, 

 the index or limb of the instrument shall pass over the 

 whole of the measured angle. 



By this contrivance any angle taken in the field may be 



at 



