266 



Scientific Geology. 



that this is a genuine case of the conversion of argillaceous limestone 

 into chert. For, says Dr. Macculloch, " originating in limestone, the 

 transition from that rock into the chert, is often so gradual, that no 

 precise point can be assigned where the term indurated limestone is 

 no longer applicable."* None of it, however, at this locality has 

 that chalcedonic aspect which highly indurated chert often exhibits. 



Jasper. 



The jasper in Newport, to which I have already referred, occurs a 

 few rods south of fort Adams. Two or three large blocks of gray- 

 wacke slate lie upon the shore, a considerable part of which has been 

 converted into red jasper, often mixed with a greenish, translucent, 

 siliceous mineral, so as to form an imperfect heliotrope. Before the 

 mouth blowpipe the greenish variety undergoes no change, except a 

 loss of color : but the red variety loses its color, and becomes slightly 

 glazed at the surface. (Nos. 387, 387 1-2.) 



The manner in which this jasper has been produced, appeared to 

 me quite obvious : indeed, if I mistake not, we see the various steps 

 of the process. The rock contains a considerable quantity of the 

 magnetic oxide of iron ; which, according to the latest writers,! con- 

 sists of one atom protoxide and two atoms of peroxide. Now the 

 effect of heat would be to convert this moiety of the black protoxide 

 into the red peroxide ; and the iron would serve also as a flux for the 

 fusion of the slate ; and thus genuine jasper would be produced : for 

 according to Mohs, " jasper, with its various kinds, is formed, if be- 

 sides the oxide of iron, clay enters into the mixture, &c."| Those 

 parts of the rock with which the iron did not mingle, would form 

 hornstone or heliotrope ; the latter deriving its green color from the 

 slate, which has a greenish aspect. As the vicinity of this locality 

 exhibits so many marks of the former action of powerful heat, can 

 we doubt that in this manner the jasper was produced ? and can we 

 doubt but granite was the powerful igneous agent employed ? 



Saugus has long been known as a locality of fine jasper. It ex- 

 hibits a blood red, and generally uniform color ; though sometimes 

 striped and clouded with white, so as to be very elegant when pol- 

 ished. (Nos. 388 to 390.) I have been surprised, however, to find 



* System of Geolog-y, Vol. 2. p. 284. 



t Thompson's Chemistry of Inorganic Bodies, Vol. 1. p. 487. Edinburgh, 1831. 

 t Mobs' Mineralogy, Vol. 2. p. 328. Edinburgh, 1825. 



