172 Scientific Geology. 



bury. Similar beds of shells are also found on Nantucket : such as 

 Natica, Pyrula, Venus, Crepidula, Solen, Pecten, Area, &c. as will 

 be more particularly described when I treat of Plastic Clay. I am 

 not certain that in any of these cases the shells occur in diluvium 5 

 though I regard that as the most probable supposition. 



3. TERTIARY FORMATIONS. 



For a long time these formations were confounded with alluvium 

 and diluvium : but they are clearly distinguished from both, by the 

 much finer state of most of the materials that compose them ; by the 

 greater regularity of their stratification ; by their relatively inferior 

 position, and by containing peculiar organic remains. As appears 

 from the map attached to the recent geological work of Mr. Lyell,* 

 tertiary strata occupy more than half of the surface of Europe ; yet 

 geologists had paid very little attention to them till the publication of 

 the work of Cuvier and Brongniart, on the Paris Basin, in 181 1. In 

 our country, although these formations occupy a vast extent of sur- 

 face, particularly in the southern States ; embracing that broad tract 

 along the coast marked on Mr. Maclure's Map as alluvial ; yet have 

 they received but very little elucidation. Messrs. Morton, Vanuxem, 

 and Conrad, have, however, recently devoted themselves successfully 

 to this subject. 



After the tertiary beds around Paris and London had been des- 

 cribed, it seemed for a long time to be taken for granted, that tertiary 

 strata all over the world must be identical with these : as if those 

 spots contained the types of the whole globe. But geologists now 

 find that no formations are more independent than the tertiary ; and 

 that it is very difficult to ascertain a precise identity of origin of any 

 two basins, even when near to one another ; and as to those that are 

 widely separated, it is no easy matter to determine whether they were 

 deposited during the same geological epoch. 



I shall describe the tertiary rocks of Massachusetts under two di- 

 visions: 1. The most recent tertiary; and 2. the Plastic Clay. 

 These are distinguished from each other by their mineralogical 

 characters, their organic remains, and the different position of their 

 strata. 



* Principles of Geology, &c. by Charles Lyell, Vol. Q. London, 1832. 



