B. K. Emerson — The Deerfield Dyh and its Minerals. 201 



The cellular or excavated structure occurs frequently and can 

 be produced in fresh specimens by acid, by the removal of 

 calcite. The occurrence of the mineral in fresh prehnite and 

 calcite would militate against its being here a pseudomorph 

 after siderite. 



Products of the decomposition of diabantite.— From its great 

 content of ferrous oxide the mineral is very prone to decompo- 

 sition and in sections cut near the weathered surface of the 

 rock its color has become red-brown, its strong dichroism is 

 gone, and it shows only the faintest aggregate polarization; 

 Sometimes a cavity shows the exterior changed and brown, the 

 interior bright green, with sharp lines of demarkation. In the 

 large amygdules masses of gold or bronze-yellow, sometimes 

 light straw-yellow, can be obtained, which extol ue vo mmn- 

 ously under the blowpipe and then by a sharp explosion throw 

 off the exfoliated portion, and begin the operation anew. It 

 is a diabanlite-vermiculite. At the end of this series of changes 

 only a small quantity of li: 



When a slide of the diabase is treated with hydrochloric 

 le fresh and the altered diabantite are decomposed 

 silica remains behind in plates having still the 

 irrangement of the original mineral. This is also 

 th the vermiculite out of the primary rocks in 



Pelham. 



Similar amygdules occur in the compact diabase having a 

 white color or being in part still green and dichroic, and hav- 

 ing exactly the arrangement of the diabantite. These are, I 

 have no doubt, silica, and have been produced by some n^™ 

 process analogous to the artificial one employed above, lhe 

 white portions show marked aggregate polarization in white 

 and black but without bright colore. 



Albite. In the cavities in the amygdaloid from the shores 



Turner's Falls, which are often only partly filled with 

 the vermiferous diabantite (vide p. 199) and out of which the 

 latter may be shaken as a deep green powder, the 



crystals loosely s 



, or in the 



powder shaken out from the cavities and scarcely 



all. Thev arc •<><)"> to ■llliiim. in diaun ■ 



fectly fresh crystals, twinned in accordance with the albite !ai\\ 



Thev are combinations of the form /-?, O, /-£', i-l A/, /' 7 ' 



2-7. llatten.Ml 1-v the large development of /-/, and 



gated in the direction of the axis a, by one end of « 



are uniformly attached. Examii 



scope the line of exti m ma - tl ■' edge <> A '-' ai 



angle of +4° to +4° 30' on the face 0. When lying upon the 



face i-l the minute twins gave varying results. The first result 



indicates that the crystals are albite according to the determm- 



