of the Southern part of Essex County. 85 



When first quarried it can be cut with a saw or turned in a 

 lathe, but in this state it will not receive so good a polish. Af- 

 ter being exposed to the atmosphere it becomes dry, much 

 harder, receives a good polish, and is a good substitute for mar- 

 ble where stones of a dark color are preferred. 



Near the surface, it is so much divided by veins of asbestos 

 and other foreign substances, as to be of little value, except for 

 the manufacture of Epsom Salts ; but at the depth of fifteen or 

 twenty feet, slabs sufficiently large can be obtained, to answer 

 all ordinary purposes. 



This bed, in all probability, is the dividing line between the 

 gneiss formation on the north west and the sienite on the south 

 east, 



From 100 parts of this rock, Dr. C. T. Jackson obtained by 

 analysis : 



Silica, -v^fS?. 1 '-" " ''':<' '-'.?-, \"* b ''''"^ 37 



Magnesia, V" : 4 . / ' : '.( ;V^ ; " .\ 42 



Oxide of Iron, , .'*] *' '1;^' vV ^" v 'i 1 '.' /".""'. ~ 



Water, . -. ; '. / v . . , . . ^ 



Loss, . . ; '. ' ^ V V* ^*:v; /: .... . . : ^ 



100 



Dr. Jackson says, that from 100 grains of the rock, united 

 with sulphuric acid, may be prepared 127 grains of dry Sulphate 

 of Magnesia, or 198 grains of crystallized Epsorn Salts. 



This will give, by decomposition with the carbonate of soda or 

 potassa, 98 grains of the common carbonate of magnesia of the 

 shops. 



In Roxbury, works have been erected where Epsom Salts are 

 now extensively manufactured from this rock. 



SIMPLE MINERALS. 



CLASS I. ORDER III, GENUS III. LIME. 

 Species I . Fluate of Lime. A few specimens at Nahant, one 

 or two small crystals of Derbyshire spar, also at Nahant. 

 6 



