594 AV. A. TARR OOLITES IX SHALE AXD THEIR ORIGIN 



gels are especially apt to assume such shapes because of the lack of any 

 cohesive force save surface tension. Surface tension serves a very impor- 

 tant part in the development of the oolites, as it tends to aggregate the gel 

 into small spherical bodies. It is this combination of much silica being 

 added to the seas, its precipitation as a gel, and the tendency of gels to 

 assume spherical forms that strongly favor the view that the siliceous 

 oolites are original. 



EVIDESCE FOR THE THEORY OF PRIMARY DEPOi<FTIOX 



The following reasons are the most important for believing that the 

 silica was deposited at the same time as the inclosing rock, which was a 

 sandy shale in this case : 



1. The uniform distribution of the oolites through the shale without 

 any evidence of accumulation along a given plane is in favor of this view\ 

 The shale itself may or may not be entirely without banding. Uniform 

 distribution is what would be expected if the siliceous material accumu- 

 lated along with the muds of the shale, both settling to the bottom as 

 they were added to the water. 



2. The uniform distribution of sand grains in both oolites and shale 

 favors the primary deposition theory. The sand grains were carried down 

 by the muds and were included in the gel of the siliceous oolites. Their 

 arrangement in the oolites shows that they were simply included in the 

 soft material and were in no ^^•ay affected by further enlargement of the 

 oolites — a fact wholly compatiljle with the growth of gels. Possil^ly some 

 of the sand grains were acquired by adhering to the gel. There is also 

 considerable kaolin scattered through the oolites, especially in the central 

 zone of some of them. This is evidently due to the central zone forming 

 soon after entering the water, Avhere it was, of course, very muddy. 



3. All growth of the oolites ceased as soon as they were buried in the 

 muds. They could grow as long as they were settling through the water 

 and until they had been ]juried by the accumulation of mud on the l)ot- 

 tom. Practically all the silica went into the oolites, for the shale is only 

 slightly plastic. 



4. The interference of the oolites is a very suggestive feature. As can 

 be seen in the plates, the oolites are frequently in contact, the line of con- 

 tact usually being regular. There is no evidence in any of the shales that 

 the inner zones have been deformed by their growth. The oolites have 

 evidently continued to grow after coming to rest on the bottom. Two 

 adjacent oolites grew uniformly and thus preserved a regular line of con- 

 tact. If a small oolite Avas in contact with a large one it Avas usually 

 partly surrounded by the larger. 



