PETROORAPHIC GEOLOGY. AND DESCRIPTIONS. 167 



Calcite. Diabase.] 



No. 90A. CAL.CITE. 



From a vein which occurs in No. 90, but splits and runs in stringers in the face of the blutf. The fracture 

 of No. 90 produced fragments which were subsequently surrounded by No. 90A. This calcite is granular and 

 massive, or occasionally spongy. 



Kef. Annual Report, ix, page 24. 



One poor section. N. H. w. 



No. 90B. DIABASE, ((.\un-w.) 



"Concretions from No. 90." 



Ref. Annual Report, ix, page 24; Bulletin ii, pages 99, 100. 



Meg. A coarse-grained rock consisting of lath-shaped, gray feldspars cutting 

 a darker substance which is mostly pyroxene. 



Mic. The section shows a coarse-grained rock composed essentially of feldspar 

 and pyroxene, with alteration products. The pyroxene is later than the feldspar, 

 the former sometimes occurring in large plates. 



The feldspar is partly fresh, though most of it is more or less altered. It is 

 commonly twinned according to the albite law. Some of the fresher grains showed 

 equal extinction angles on either side of the twinning line, running up as high as 32. 

 No grains cut exactly perpendicular to a bisectrix were found, but the extinction 

 angle would show that the feldspar is most probably labradorite. Many of the feld- 

 spars are clouded and sometimes almost opaque, being filled with small, gray, opaque 

 specks. But the peculiar feature of the feldspar is its alteration to a certain mineral 

 which occurs all through the section, and is developed in small flakes directly in the 

 feldspar grains. This mineral has a low index of refraction, a rather high double 

 refraction, is often arranged in radiating fibrous masses. One distinct cleavage, 

 parallel to the length of the fibres and also to the elongation of the plates, is present 

 and extinction is parallel to this cleavage. The elongation is sometimes positive 

 and sometimes negative. This mineral possesses the characters of thoinxon/tf and is 

 here referred to that species. 



The pyroxene is apparently avgite, but it is undergoing alteration, which pro- 

 ceeds along the parting planes. The alteration products are chlorite, hornblende 

 and a yellow substance composed of small flakes. Hornblende and chlorite, how- 

 ever, occur all through the rock independent of the augite areas. The hornblende is 

 in small sheaths and tufts of fibres which are markedly pleochroic. Magnetite and 

 apatite are quite common in the sections. 



Three sections. 



Age. Cabotian. 



Remarks. This is the first rock here described that shows the feldspars altering 

 to thomsonite. The authors know of several other occurrences of thomsonite in 

 such diabase, viz.: Nos. 106, 200A, 627A, but perhaps none in which the origin of the 



