64 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 17, 



being those on the western coast of South Africa, and the western 

 coast of South America respectively. 



The abnormal forms of the isothermals in the northern Atlantic 

 and North Sea are manifestly due principally to the warm waters of 

 the Gulf Stream ; those of eastern Asia and North America are attri- 

 butable to the existence of large masses of land in high northern 

 latitudes ; while those above-mentioned in the southern hemisphere 

 are immediately traceable to the influence of well-known ocean-cur- 

 rents setting from the south towards the equator along the coasts of 

 southern Africa and South America. The water of these currents 

 reduces the temperature of those parts of the ocean through which 

 they pass, and consequently also that of the superincumbent atmo- 

 sphere, and thus causes the isothermals to deviate to the north of 

 their normal positions. Thus an examination of all the principal 

 deviations of the isothermal lines from their normal types leads to 

 the conclusion that ocean-currents and the configuration of the great 

 continents are the principal general causes which produce irregulari- 

 ties in the forms of those lines ; and, moreover, a knowledge of these 

 causes enables us to assign to the isothermals their approximate po- 

 sitions in any proposed hypothetical case in which the disposition of 

 land and sea should be different from that which obtains at the pre- 

 sent time. 



The different hypothetical cases for which I shall endeavour to 

 determine the isothermal lines are the following : — 



(1.) The configuration of land and sea the same as at present, but 

 without the Gulf-stream. 



(2.) The Gulf-stream the same as at present, except that its pro- 

 gress into the North Sea is supposed to be arrested by a barrier of 

 land, extending from the north of Scotland to Iceland and thence to 

 the coast of Greenland. 



(3.) The basin of the Atlantic from the Tropic to the North Sea 

 converted into land, uniting the old and new continents. 



(4.) Large portions of the continents of Europe and North Ame- 

 rica submerged beneath the surface of the ocean, and the Gulf-stream 

 diverted into some other course. 



The consideration of these cases will occupy the first Section of 

 this Part of the memoir. 



10. The snow-lint, or that line on the side of a mountain above 

 which the snow never disappears at any season of the year, bears an 

 important relation to glaciers, since it divides that higher region, 

 in which productive agencies prevail in augmenting superficially the 

 mass of a glacier, from the lower region, in which the destructive 

 agencies predominate. It is essential to our investigations to know 

 the vertical distance which existing glaciers usually descend beneath 

 the snow-line, that we may be able to judge by analogy of the pro- 

 bable distances to which ancient glaciers may have descended. 



As we ascend from the surface of the earth, the mean annual 

 temperature decreases according to laws which have been approxi- 

 mately determined by observation. If this temperature, therefore, at 

 any proposed place be greater than 32° F., we shall only arrive at 



