Geological Society of Glasgow, 143 



plates which passed inwards and downwards, conforming to the 

 central concavity of the top of the columella. 



n. — January the 9th, Prof. Young, President, in the chair, the 

 Secretary read a letter from Mr. Eobert Craig, of Beith, on the 

 Glacial Drift in that district. He stated that it consisted of two 

 easily distinguished beds, the lower composed of dark-blue clay, full 

 of polished stones of great size ; the upper of a light reddish friable 

 clay, full of small stones, rounded and smoothed, but the polish had 

 a weather-worn appearance. 



The Eev. Mr. Crosskey read some "Notes on the Discovery of 

 Leda arctica at Stevenston, Ayrshire." This shell is so remarkable, 

 on account of its high northern habitat, that its discovery in any bed 

 is of peculiar importance. Living, it dwells only within the Arctic 

 circle and on the North-east Coast of America. It was identified by 

 Dr. Torell among the shells found at Elie by the Eev. T. Brown, 

 and occurs in the clays at Errol and Montrose, on the east of Scot- 

 land, but hitherto has not been found in any of the clay beds of the 

 West. The specimen exhibited (kindly given to him by Mr. 

 Armstrong) was found at Stevenston, Ayrshire, and is in a fine state 

 of preservation. Its occurrence in the West proves that the 

 character of our fauna has been as severely Arctic as the ancient 

 fauna of the Eastern beds. ' Leda arctica is the characteristic shell 

 of the clay at Mess in the Christiana fjord, although not now found 

 living even in that locality. It is also a characteristic fossil in the 

 Saxicava-sand of Canada. The occurrence of Leda arctica in the old 

 clays of Canada, Christiana fjord, Errol, and Stevenston, affords a 

 curious illustration of the wide distribution of very highly Arctic 

 moUusca during the glacial epoch. Mr. Crosskey also read a paper 

 " On the Characteristics of Boulder Clay." There are various 

 boulder clays, instead of one single deposit, which it is imperatively 

 necessary to distinguish from each other. The attention of geo- 

 logical students was called to the necessity of discriminating between 

 the different kinds of materials which have been loosely united under 

 the general name " boulder-clay." 



III. — Thursday evening, 16th January, E. A. Wiinch, Esq., V.P., 

 in the chair. 



Edward Hull, Esq., E.E.S., of the Geological Survey of Scotland, 

 delivered a lecture " On the physical causes which seem to have 

 regulated the distribution of the calcareous and sedimentary strata of 

 Great Britain, with special reference to the carboniferous formation." 



While admitting that calcareous matter was sometimes precipitated 

 on the sea-bed from chemical solution, the lecturer maintained that 

 all great masses of limestone are the result, either directly or indi- 

 rectly (by subsequent triturition and stratification) of vital agencies, 

 while the sedimentary strata were of mechanical origin. Hence the 

 essential distinction of the two classes in their origin. It was shown 

 that the limestones of geologic periods were the work of a few 

 groups of animals of an organization inferior to that of the molluscs, 



