PRINCIPLES ZOP ASTRATIGRAPHY 
  
718 
by David White (24), by the degree of injury suffered by the 
perishable parts, such as bark, leaves, etc. Seeds and spores of river 
plants, such as the water ferns or rhizocarps, may also be carried 
out to sea and buried in normal marine sediments. Their occur- 
rence, however, always suggests a river-borne origin for: the mud 
in which they are found, as in the case of the Genesee and Portage 
muds of the Devonic in which occur the spore cases of the water 
fern Protosalvinia, mingled with marine organisms. 
16. ConcrerioNs. Concretions are segregations of mineral 
matter which grow in size by addition externally, internally or in- 
terstitially. From the point of view of their origin and relation- 
ship to the enclosing rocks, two types may be distinguished: (1) 
those forming as contemporaneous accumulations, afterward buried 
by clastic or other strata, and (2) those forming within the strata 
after their deposition. This second group clearly belongs to the 
secondary structures of rocks. 
Concretions of calcium carbonate, of barite, of manganese, and 
concretions composed of fragmental material cemented by phos- 
phate of lime are among the first group, forming at the present 
time. The phosphate concretions are most characteristic of the 
shore zone, while the manganese concretions are common in the 
deep sea. The latter (Walther-23: 70r) are most abundant in the 
Pacific between 767 and 8,183 meters, and in the Indian and 
Antarctic Oceans between 2,026 and 4,754 meters depth. In the 
Atlantic Ocean they are found between 767 and 5,211 meters depth, 
chiefly in the neighborhood of volcanic islands. 
These concretions commonly constitute the uppermost layer of 
the lithosphere in the deep sea, and they are gradually buried by 
the accumulation of fine muds. Not infrequently they constitute 
the foundation on which corals or other sedentary benthonic organ- 
isms gain a foothold, and such a concretion in moderately deep 
water may serve as the nucleus about which a coral reef is built up. 
Chemically formed oélites and pisolites should be mentioned under 
contemporaneous concretions. These have been fully discussed 
in a preceding chapter. ' 
The secondarily formed concretions, or those growing within 
the strata, and therefore of later age, are represented by clay-stone 
concretions, so characteristic of shale and clay beds, and readily 
recognized as belonging to this secondary type by the fact that the 
stratification lines are seen to pass through them. ILarge examples 
of these are found in Devonic and later lutaceous deposits. They 
have not infrequently grown about fossils. Many of these also 
show the septarium structure as described below. The Losspiippchen