26 REPORT OF NAT J ON AL MUSEUM, 1889 



(72959), aud the smaller slabs from Huinmelstown, Pennsylvania (27055), 

 are then but casts in sand of these old cracks (Geikie, p. 471). The 

 rain prints shown on the small slabs of Triassic sandstone from New 

 Jersey indicate that the stone while still plastic was exposed to the pelt- 

 ing action of a shower, the drops leaving their imprint in the soft mud. 



Footprints : It has not infrequently happened that animals wading in 

 the shallow water left footprints in the mud to be covered and preserved 

 in the same manner as were the cracks and ripple marks above referred 

 to. But few of these are shown here, since the subject belongs more 

 properly to vertebrate paleontology. On the large slab pinned against 

 the south wall are two consecutive tracks of the Brontozoum gigartteum 

 (H), a huge reptile estimated to have been at least 14 feet in height 

 aud which inhabited the Connecticut Valley during the Triassic period. 

 The smaller slabs pinned high against the east wall show tracks of 

 Brontozoum validhtm and SiUimanium, and Anomcepus cuneatus. Qu the 

 small slab from South Hadley Falls, Massachusetts, are shown the mud 

 nests of tadpoles, Batracho ides nidifict ens of Hitchcock. 



A large slab of Potsdam quartzite from New York showing faintly the 

 trail of a marine mollusk (Protichnites Loganns), made as he crawled 

 slowly over the soft bottom of the Cambrian ocean, serves further to 

 illustrate the conditions under which these rocks were formed. 



Concretions : The peculiar tendency which atoms or particles of like 

 matter often manifest in concreting or gathering in concentric layers 

 about centers is shown by a large and diversified collection of concre- 

 tious. As here arranged these are divided into two groups, as follows: 

 (A) Primary concretions, formed contemporaneously with the rock in 

 which they are found, and (B) secondary concretions, or those which 

 are due to segregating processes acting subsequent to the formation of 

 the rocks in which they are found. Each of the groups may be subdi- 

 vided accordingly as the concretions were formed as chemical precipi- 

 tates or are but aggregates of mineral particles bound together by an 

 interstitial cement. 



(A.) Primary concretions : (a) chemical deposits and (b) mineral ag- 

 gregates. 



Under (a) are here included the chalcedonic nodules found in lime- 

 stones (specimens 38434, 38435, and 37603), the pyrite concretions, such 

 as No. 39053, aud the clay ironstones, such as Nos. 12890 and 37303 ; 

 these last are often found to have cracked interiorily on drying and 

 consequent shrinkage, and the cracks to have become subsequently 

 filled with carbonate of lime. On being cut and polished such ofteu 

 form beautiful and unique objects, as shown in the specimens from 

 Kansas (12890), Indiana (25100), aud New York (39129). To such forms 

 the name Septarian nodule is commonly given. Here also are displayed 

 the fine oolitic and pistolitic concretions such as those of Bohemia (30096 

 aud 36097), Hungary (36099), Cache Valley aud Salt Lake, Utah (35305 



