300 W. M. Hutchings — Sedments from Lakes. 



is to say, up to the Concavum-zone, inclusive, is " Lias," above it 

 " Jura." It may be remarked that no local lithological details, 

 whether in the Central Apennines or in this country, can have the 

 least consideration in regard to the division ultimately adopted. 

 It must be settled solely on palEeontological data; and the 

 following facts may be noted, that, below the Concavum-zone, the 

 Arietidcd and Hildoceratidm (practically the carinate Ammonites) are 

 dominant; after that date the Stephanoceratidce hold the field. This 

 distinction is of European, and possibly of worldwide, application ; 

 while a lithological test is of merely parochial usefulness — sometimes 

 not even that. 



V. — Note on Sediments Dredged from the English Lakes. 

 Ey W. Maynard Hutchings, F.G.S. 



DE. H. C. MILL has recently made elaborate bathometric 

 surveys of the principal English lakes, and whilst taking the 

 soundings he has collected a series of samples of the bottom-deposits, 

 for the purpose of microscopic study. 



At the request of Mr. Marr I undertook a careful mineralogical 

 examination of these samples, and, though nothing very striking 

 or specially interesting has resulted from the work, it may be 

 worth while to record in the Geological Magazine the fact of such 

 an examination having been made, and to state the mineralogical 

 and other observations noted. 



After some preliminary experiments, the method of dealing with 

 the sediments finally adopted was as follows. Each sample was 

 passed through a sieve of 130 holes per linear inch, and only the 

 material fine enough to pass this mesh was submitted to microscopic 

 examination at all. From this first portion a second was separated 

 by a sieve of 250 holes per inch, and from this again a fine slime 

 was washed out by levigation in beakers ; also a separation of the 

 heavier minerals of the deposits was made by means of a solution 

 of Sp. G.=3. The four grades of material so obtained were ex- 

 amined mounted in balsam and in water. 



I have been much struck by the fact that most of the samples 

 show great tenacity when dried, and great plasticity when made 

 into a paste with water. This high degree of tenacity and plasticity 

 was unexpected, in view of the large amount of quite angular and 

 gritty material contained, and the relatively small proportion of 

 mineral matter in a sufficiently fine state of division to be compared 

 to the ordinary clays. Partly, however, these qualities are due to a 

 considerable amount of organic matter of vegetable origin, and to 

 abundant diatoms and other mineral organisms and their remains. 



The mineralogical composition of the deposits is, in the main, 

 exactly what would be expected from a knowledge of the rocks in 

 the respective areas drained, though there are one or two examples 

 of minerals which occur in the sediments in greater abundance than 

 would have been looked for, as e.g. hornblende, both green and 

 brown, occurring in lakes receiving the drainage of areas of 



