10 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



auce. Iii 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 

 Hints, 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 hud 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- 

 donic silica so perfectly transparent that they have a hyaline appearance. 

 The remains of Protozoans in the Hint, as has I teen 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-stellati' was found, and that one had large centre and short 

 straight spines somewhat similar to Figure 1G of these flints. One sili- 

 ceous ball was found, but none of the smaller dermal spicules, although 

 acuatcs. trifids, quadradiates, etc. of every size and form were present 

 in great numbers, crowded together in heterogeneous masses. The dif- 

 ferences between the chert and the Hint seem difficult of explanation, 

 unless it lie that the chert was formed more rapidly than the Hint, aud 

 there was less time fur solution and solidification. 



