292 PROCEEDINGS OP TFIE AMERICAN ACADEMY 



The augite is apparently fresh, and is very subordinate in (juantity 

 and size of individuals. Next, as regards quantity, to the feldspar, is 

 the soft, green mineral. It is in rather rounded grains, suggesting rec- 

 tangles with the corners rounded off, and elongated hexagons. The 

 contours are not broken by the feldspar or augite crystals, from wliich 

 they would seem to be the oldest constituent. They are strongly 

 marked by thick, parallel, transparent, colorless lines, indicating open 

 cleavage-cracks, show strong absorption for intensity, and become dark 

 between crossed nicols when these lines are parallel to one of the nicol 

 undulation-planes. Many were seen which showed no parallel lines, 

 and these were, probably, cut parallel to the plane of cleavage, but 

 none of them revolved dark between crossed nicols ; the mineral is, 

 therefore, probably orthorhon]bic. There can be little doubt that this 

 mineral is pseudomorphous after olivine. Its contours are identical with 

 those of the olivine in 108. The parallel structure is there represented 

 by the tendency to a fibrous structure ; and both the alteration product 

 and the fresh olivine of the same individual are dark, when the direc- 

 tion of these fibres is parallel to one of the nicol undulation-planes. 



There are also present many small pseudo-amygdules, filling the 

 wedge-shaped interstices between the feldspars. 



The annexed analyses of specimens from the three members of bed 

 No. G4 serve to throw some light on the primary mineral constitution 

 of the rock, and on its alteration. The calculation of the primary 

 minerals in the fresher, bottom rock, can be only roughly proximative, 

 while the optical measurements indicate a predominance of anorthite. 

 The analysis confirms this, and points to the presence of a little soda, or 

 soda-lime feldspar. 



In the analysis from the middle, or amygdaloidal region, we find a loss 

 of one-third of the soda ; and a gain in lime, which marks the altera- 

 tion of the feldspar to prehnite. The potash belongs to the orthoclase, 

 which is pseudomorphous after prehnite. 



The analyses were made for me by Mr. "Woodward, of the Sheffield 

 Scientific School. 



