v b 



X^CAUFO*^ 



SACCAM.MINA CAIITKRI. '50 



posed of spheroidal or fusiform bodies, hut so aggregated and infiltrated that they 

 form an intensely hard dark-coloured limestone, the freshly fractured surface of which 

 appears almost homogeneous and sometimes subcrystalliue. It is, however, readily acted 

 upon by the atmosphere ; and the weathered portions reveal a spheroidal structure that 

 might at the first glance be assigned to purely physical causes depending on sonic pecu- 

 liarity in the mode of deposit. A fair idea of the characters of the rock forming this 

 bed may be gained from Plate I, fig. 1, which represents an average specimen, with the 

 upper surface considerably weathered. Very frequently the disintegration, instead of 

 being merely superficial as in the figured specimen, extends to a considerable depth, 

 leaving the stone in the condition of a crumbling mass of spheres. A layer in this state 

 often exists between the surface-soil and the hard rock ; and by a little treatment the fossil 

 portions may be obtained from it quite clear of the matrix. A few feet below this upper 

 layer (in the same section), is a second and more considerable bed, with the same sort of 

 fusiform bodies distributed through its entire length and thickness. The individual 

 specimens arc larger than those occurring in the later deposit, but they do not constitute 

 nearly so considerable a proportion of the entire rock. The segments do not appear to 

 differ in structural characters from those found in the upper bed." 1 



Notwithstanding the black and to the naked eye almost homogeneous texture of the 

 rock as exhibited by a freshly fractured surface, a section thin enough to be transparent 

 shows exactly the structure that might be expected from the condition of the disin- 

 tegrating portions a mass of fusiform segments cut in various directions. Such a 

 section is seen in PI. XII, fig. 6. 



The dimensions of the individual segments vary considerably in different localities, 

 indeed even in the same bed specimens differing much both in size and shape may be 

 found, as shown in Plate I, fig. 3. These, however, represent an extreme range, and in 

 many rocks the chambers are exceedingly uniform in external characters averaging about 

 | inch (3-2 mm.) in length, and -^ inch (2'1 mm.) in transverse diameter: large 

 specimens may be found measuring (4'2 mm.) or even | inch (5* mm.) by (2'S mm.) 

 or |- inch (3'2 mm.); but such are of rare occurrence. Sometimes they are more 

 elongate ; and extreme examples have been noted in which the conjugate and transverse 

 diameters were in the proportion of 3 to 1. The two ends are usually produced and 

 tubular, to permit the passage of sarcode stolons or pseudopodia : they are sometimes 

 symmetrical, but more frequently one end tapers more gradually than the other. 



Owing to the tenuity of the connecting stolons, the segments are almost invariably 

 found separated. Occasionally the weathered surface of the rock reveals two or even 

 three chambers united in a line, but this is quite an exceptional circumstance. The 

 dissociation of the segments is probably entirely due to external agencies, for that the 

 addition of chamber to chamber might go on indefinitely under favorable conditions can 

 scarcely be doubted. Nor does this rest on mere assumption, as reference to Plate I, fig. 4, 

 1 Ann. and Mag. Nat. Hist., 4 ser., vol. vii, p. 178. 



