10 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
ance. In spicules replaced by peroxide of iron, or wholly by amorphous 
silica, the canal is not usually shown. An exception is found, however, 
in Figure 5, in which the canal is replaced by the ore. 
Comparison with English Chert. 
During the progress of this work I have been so fortunate as to receive 
from Dr. Robert T. Jackson a piece of chert collected in England by 
Dr. C. E. Beecher, of New Haven, Connecticut. "This chert, I am in- 
formed by Dr. Beecher, was collected at Croydon, England, but the 
geological position was not stated, and its exact horizon is not known. 
I thought, however, that a comparison of this with the Texas flint might 
give a clearer notion of the character and composition of the Texas 
flints, especially to those who have studied the cherts of England. The 
chert is a light brown to dark brown color, which contrasts strongly with 
the dense black of the flints; it is much softer, and contains numerous 
small patches, and some large areas of crystalline calcite. The appear- 
ance in the microscope is quite different on the whole, although similar 
in some respects. The chert did not have that granular amorphous 
silica so common, as did the flints, but instead it seemed to be composed 
of a dense aggregation of spicules so entangled and interlaced that it is 
almost impossible to find a spot where there are not dozens in the field 
of the microscope. 
In the chert the spicules are much more perfect in form, and, as a 
rule, they are replaced by a crystalline silica or peroxide of iron. Sev- 
eral cases were noticed where the entire replacement was by a chalce- 
donie silica so perfectly transparent that they have a hyaline appearance. 
Тһе remains of Protozoans in the flint, as has been said, are few, and are 
replaced chiefly by arenaceous, amorphous silica; while in the chert, 
protozoans, chiefly foraminiferous, are abundant, and are generally 
replaced by chalcedonic silica. 
In the whole piece of chert (four slides were examined) only one 
globo-stellate was found, and that one had large centre and short 
straight spines somewhat similar to Figure 16 of these.flints. One sili- 
ceous ball was found, but none of the smaller dermal spicules, although 
acuates, trifids, quadradiates, etc. of every size and form were present 
in great numbers, crowded together in heterogeneous masses. Тһе dif- 
ferences between the chert and the flint seem difficult of explanation, 
unless it be that the chert was formed more rapidly than the flint, and 
there was less time for solution and solidification. 
