400 



ANALYSIS OF LIMESTONES. 



Beds of limestone rarely decompose beneath the soil. Water percolates into the joints 

 and enlarges the planes of separation by gradually dissolving portions of the carbonate of 

 lime. This process has been exemplified at the west end of the tunnel, just mentioned, 

 where the work of excavation has been greatly hindered in consequence. The fragments 

 may assume imitative forms ; and when particles of mica, etc. are numerous in the rock, 

 the removal of the limestone leaves them prominently upon the surface. 



III. MAGNESIAN LIMESTONES AND DOLOMITES. 



It is not always easy to distinguish dolomites from limestones by external appearances. 

 The chief distinctions are these : 1st, The texture of a dolomite is less firm than that of 

 pure limestone; so much so that it may crumble into sand. 2d, Dolomite is less dis- 

 tinctly stratified than pure limestone. The great mass of dolomite in the red sandrock 

 group illustrates this distinction more forcibly than the dolomites of the present group. 

 3d, When pure limestone in the state of powder is mixed with nitric acid, it dissolves 

 rapidly, with powerful effervescence ; so that in a few moments, if enough acid has been 

 added, nothing remains undissolved except the earthy residuum. But dolomite does not 

 effervesce so briskly, hardly showing any agitation, and yet requiring a long time for its 

 complete solution. When carbonate of lime is mixed with dolomite, both actions are 

 seen ; but the first is speedily accomplished, and the second continues to act for a long 

 time as before. In many cases, however, only a careful analysis will decide the character 

 of the rock. The annexed table gives the analysis of all the dolomitic Eolian limestones 

 that have been made for the Survey : 



Locality. 



Limestone, Bristol, 

 Brecciated limestone, Bristol, 

 Water limestone, Shelburne, 



Yellowish limestone, Brandon, 



Thus, generally, these varieties are either a double carbonate of lime and magnesia, or a 

 carbonate of lime mixed with a large proportion of the double carbonate, and where 

 magnesia exists at all in limestones, it probably exists in combination as a dolomite, the 

 remainder being carbonate of lime or carbonate of magnesia, in excess. The dolomites 

 are generally crystalline, often finely granular and sometimes pulverulent,. but rarely com- 

 pact. They decidedly predominate along the eastern part of the limestone range, and 

 are sometimes interstratified with pure limestone. It is a noticeable fact that the pure 

 limestones abound at the western edge of the deposit, and the dolomites with silicious 

 limestones at the eastern limit. Circumstances were varied at the different periods when 

 the several varieties were produced. We are not certain whether the magnesia was all 

 introduced when the rock itself was produced, or whether the process of alteration in 

 some way has introduced the magnesia. 



The dolomite is sometimes fetid, so as to give a strong odor when struck with a ham- 

 mer. This is usually regarded as proof of the existence of organic matter. Some of the 

 layers of dolomite of this range, in Massachusetts, are flexible when moistened, especially 

 in New Ashford. 



