1897J MICROSCOPICAL JOURNAL. 281 



London clay. It is from beneath the Red Crag which is 

 called the older Pliocene. There is no Miocene, which 

 belongs between these, and we expect to find the diatoms 

 very different. The forms that exist in the Pliocene are 

 about the same that grow now. The Pliocene diatoms 

 of England have not been studied. Some Pliocene are 

 enumerated in a paper on the Diatomaceous deposit of 

 the mud of Milford Haven and other localities, by Fitz- 

 maurice Okeden, in Vol. Ill, 1855. The celebrated 

 Glenshira sand which is described in the same volume is 

 most likely correlative with the Champlain clay, the 

 Raised Coast Period of our shores, judging from the dia- 

 toms in it. 



The London clay consists of a brown or bluish grey 

 clay, containing layers of concretions called septaria, — 

 "flattened nodules of calcareous clay, iron stone or other 

 matter, internally divided into numerous angular com- 

 partments by articulating fissues which are usually filled 

 with calcareous spar and show well against the darker 

 matrix of the nodule." Now that we know something 

 about the power of calcareous matter to replace siliceous 

 in organisms, for which we are indebted to the researches 

 of Sollas, Hinde, Zittel, Hill and Jukes-Browne, we can 

 reason as to what septaria are or were. Most likely they 

 were siliceous sponges. One author thinks iliat "the 

 reticulating fissues or septa (hence septaria) seem to have 

 arisen from shrinkage of the mass while in the act of 

 consolidation, and to have been subsequently filled by 

 infiltration. Such argilaceous, calcareous, and ierruf^-in- 

 ons nodules are' common in many clays and mails, as in 

 the shale of the coal formations, in the Oxford chiy, in the 

 London and Barton clays. They are often arranged in 

 lines and bands; are always more or less flattened; gen- 

 erally oontain some central organic nucleus round which 

 the matter has aggregated, such as a leaf, scale, coprolite 

 or the like ; and when split up in the direction of the 



