and South Central Pennsylvania. ±13 



oscillation rather than the current type. The nature of 

 the ripple-mark, together with the thin mud-cracked 

 layers and associated edgewise glomerates, all indicate 

 that the lower 30 feet of the Carlim were deposited in 

 relatively shallow water and that at times the beds of 

 ooze were exposed to the direct action of the sun and 

 air, the edgewise glomerates being probably formed by 

 the rising of the tide upon the previously mud-cracked 

 flat. The presence of only a few Tetradia-bearing beds 

 in the lower Carlim, and the fact that they become in- 

 creasingly abundant toward the top of the formation 

 show either that the water was becoming slightly deeper, 

 causing the bottom to become less agitated, or, that the 

 Tetradia were gaining firmer hold upon the low-lying 

 platform and gradually creating a broad-bedded reef 

 more and more resistant to the waves. It appears as if 

 the latter possibility were more in accord with the field 

 data than the former. 



In the section at Bellefonte, the middle and most of 

 the upper Carlim, except for the final 10 feet, contain 

 very few Tetradia, but Bryozoa are abundant and ap- 

 pear to have formed a sea-mat upon which a rich fauna 

 of trilobites flourished. 



In the upper Carlim of the section at Loysburg, both 

 Grirvanella and Tetradia occur in solid beds, giving this 

 part of the formation a decidedly "reef " aspect. Men- 

 tion should also be made of the occurrence of the com- 

 pound coral, Columnaria sp.f (closely related to C. 

 halli), which is found associated with Maclurites logani 

 just below the Beatricea zone. This coral has been 

 found only in the sections from Center Hall to the west. 

 In the section between Center Hall and Eeedsville it is 

 particularly abundant, heads from 2 to 3 feet across 

 being closely packed together in their original position 

 of growth; in fact, at this particular spot one would 

 have no difficulty in believing that he was looking at a 

 fossil coral reef. Columnaria is not, however, particu- 

 larly abundant in the other sections, and was not found 

 at all at Eoaring Spring and Loysburg. The Carlim 

 varies considerably throughout the area, but in all sec- 

 tions it is particularly characterized by the presence of 

 such colonial and gregarious organisms as Tetradia, 

 Bryozoa, and Girvanella. That these variations are 

 dependent upon geographic position will be more clearly 



