412 R. M. Field — Middle Ordovician of Central 



distinct as is their mode of origin and it is upon the discovery 

 of their origin that the solution of the problem depends. One 

 has bnt to glimpse at the literature on the subject in order to 

 appreciate the necessity for a reasonably accurate nomencla- 

 ture." 



The lowest bed of the Carlim was laid upon a consoli- 

 dated surface. There is no evidence of a basal conglom- 

 erate, but the presence of ripple-marks and sun-cracks 

 is conclusive proof that the pure calcareous oozes of the 

 Carlim were deposited under alternating conditions of 

 quiet and agitation. If these basal Carlim sediments 

 had been deposited upon the unlithified Loysburg sedi- 

 ments and under the conditions of agitation which have 

 just been cited, we should expect to find an intimate 

 mixture of the two types near the base of the former; 

 such, however, does not prove to be the case. The dis- 

 tinctly banded character of these rocks, together with the 

 frequent zones of ripple-marked, sun-cracked and glom- 

 eratic limestone, are particularly suggestive of shallow- 

 water and tidal conditions. The intraformational 

 giomerates represented by B 1 on fig. 2 are well exposed 

 in the sections at Bellefonte, Roaring Spring, and Loys- 

 burg, but in the former section there does not appear to 

 have been much bottom agitation and the glomerate is 

 either the direct or indirect result of organic activity. 

 The Carlim here begins almost immediately with numer- 

 ous thin beds of Tetradium, and the growth as well as 

 the destruction of this peculiar and little understood 

 organism has functioned to an important degree in the 

 formation of a typical bioglomerate* The evidence pre- 

 sented in this section indicates that at times conditions 

 were quiet enough for the growth of a thin veneer of the 

 colonial organism, Tetradium syringoporoides, but that 

 sometimes the bottom was sufficiently agitated to break 

 up the colony, together with the cementing muds, and 

 deposit the whole material in the form of a fine-textured, 

 intraformational conglomerate in which are also found 

 the remains of other attached and vagrant benthonic 

 species. In the section at Loysburg there are first a few 

 inches of light-colored, banded limestone, then a thick 

 zone of mud-cracked, ripple-marked limestone and glom- 

 erate beds which alternate with thin zones of T. syringo- 

 poroides. The ripple-mark is quite even, with an 

 amplitude of not more than 1 to 2 inches ; moreover, it 

 is decidedly symmetrical and appears to belong to the 



