MERRILL: FOSSIL SPONGE SPICULES. 23 



referred to, and also account for the broken condition of the peripheral 

 spicules. This would also account for the fact that each nodule had a 

 prevailing number of spicules peculiar to itself, while a few were common 

 to all. The peculiar form and size of the nodules may also receive an 

 explanation here. If the sponge takes root in the ooze of the ocean 

 bottom and becomes firmly embedded, there will be at its bottom a con- 

 siderable cavity where the bottom part dies. We have no means of 

 knowing how rapidly the oozes accumulate, but if they accumulate as 

 rapidly as the dissolved silica accumulates, then it would seem that the 

 ooze might enclose a pocket of the silica, having grown up around the 

 base of the sponge. In this way, the flint nodule would grow as the 

 sponge mass may have been expected to grow ; namely, it would begin 

 small, reach a maximum size, then decrease in size, and finally end in a 

 point as it began. The shapes of the flints that I studied indicate that 

 such a growth may have taken place. The consolidation of the silica 

 into flint and the ooze into chalk may have taken place about the same 

 time, but the condition of preservation of the spicules indicates that 

 there was very little pressure applied to the spicule before the consolida- 

 tion into masses so hard that pressure would not change the structure of 

 embedded fossils. 



Mr. John Murray 1 says that in the deep sea at present many of the 

 Foraminifera gather around them spicules of sponges as shells. The 

 Pilulina, etc., are covered with tests entirely constructed of cemented 

 sponge spicules, and suggest that this may be the beginning of a con- 

 cretion of flint or flint nodule. This, as is true also of several other 

 suppositions, is a plausible theory, but it seems that the small delicate 

 spicules of the Texas flint nodule could hardly have been preserved so 

 perfectly where the amount of movement necessitated by such a process 

 had taken place. 



Hence, while recognizing that my conclusion as to the process of 

 formation of the Texas flint nodules leaves several points unexplained, 

 and although I realize full well its incompleteness, yet I think it accounts 

 for more of the facts than any hypothesis I have seen. 



Summary. 



The variety and mixture of the different kinds of sponges named in 

 the preceding pages make it difficult to tell anything about the depth 

 of the ancient cretaceous sea. Geodia, which is so fully represented, 



1 Report of H. M. S. " Challenger," Volume on Deep-Sea Deposits, p. 203. 



