THE NATURAL HISTORY SURVEY, 



on the domes and base. A photograph, PL V, fig. I, natural size, 

 of a specimen from the limestone quarry at Bridgeport shows 

 a drusy cavity in the limestone full of that form of marcasite 

 known as spearhead pyrites. Fig. 8 represents the twinning 

 which produces this form. The twinning plane is the prism. 

 The prism planes are present only as an edge, while the clino- 

 dome planes are repeated until they produce disk-like groups 

 resembling a spear. In PI. V, fig. 2, the two upper specimens are 

 groups of the form known as " comb pyrites." As seen in Fig. 

 9, these are groups of twins whose twinning plane is the prism 



Fig. 8. 



Fig. 9. 



repeated in stair-like arrangement. In the lower figure of PL V, 

 fig. 2 are globular masses, very often occurring in unworn 

 and undecomposed specimens. The one at the right is on 

 Niagara limestone. The round velvety looking masses are 

 composed of bunches of crystals of different lengths which 

 extended into the mother solution. The advancing front of 

 such a growing crystal group would naturally be globular, since 

 such a form would give each crystal the largest area of solution 

 from which to draw material. The persistency of the spherical 

 habit of these concretions is illustrated in PL VI, fig. I. When 

 found on the lake shore these concretions may easily be 

 mistaken for ordinary pebbles, but when a trace of the origi- 

 nal crystal shape remains, as seen in the left hand lower 

 specimen, the true nature of the sample is recognized. Fur- 

 ther, these seeming common stone pebbles are much heavier 

 than an ordinary pebble. Upon being broken open, the yellow 

 metallic luster and the common radiate structure shown in PL 

 VI, fig. 2, attract the attention. In the lower nodules the remnants 

 of crystals left after the rolling and grinding of waves of the 

 lake shore, and after the oxidation to which the nodules have 

 been subjected, are still to be detected. The upper two speci- 

 mens show the radiated columns and foundation from which 

 they sprang. The center of the nodule is composed of fine 



