98 



SCIENCE. 



LN. S. Vol. V. No. 107. 



sheet can be solved by aid of a solution 

 for tbe case wherein the walls are kept at 

 a fixed temperature. For the temperature 

 at any point of the affected zone is the 

 same as an average temperature obtained 

 from two points as follows : If we imagine 

 a hypothetical dike of the same breadth as 

 the zone affected in the original sheet, and 

 conceive of its walls being kept at a con- 

 stant temperature, and if we imagine the 

 two points situated at the same distance 

 from the walls of the dike as was the 

 original point whose temperature was 

 sought in the original sheet, the average of 

 the temperatures of the two points will 

 be the value sought. The sum will be 

 used when the original point lies within 

 the hypothetical dike, and the difference if 

 it lies without. We need, therefore, to 

 consider in detail only the case where the 

 sides of the sheet are kept at a fixed tem- 

 perature. 



(b) Taking therefore this case of a sheet 

 originally of a uniform temperature we can 

 divide the cooling into three periods : 



1. Before the center has cooled appreci- 

 ably. During this time the time of cooling 

 is as the square of the distance of the mar- 

 gin, and is otherwise independent of the 

 sheet. 



The augite of the Keweenawan ophites 

 follows in its grain this law, the average 

 area of cross sections being proportional to 

 the square of the distance from the margin 

 and independent of the size of flow. Con- 

 solidation in this period may be expected 

 to be especially characteristic of effusive 

 rocks. 



2. While the center is cooling down one- 

 fourth of original difference in temperature 

 between sheet and margin. This period is 

 about four times as long as the first. 



3. Thereafter the rate of cooling when a 

 given temperature is reached will be inde- 

 pendent of the position of a point. Hence 

 the grain will be uniform and the same for 



all parts of the sheet that consolidate in 

 this period. The solidification will tend to 

 to fall into this period for high initial tem- 

 peratures of the magma and hot walls, com- 

 pared with the temperature of solidification 

 and broad contact zones. Hence solidifica- 

 tion in this period may be taken as typical 

 for abyssal rocks. 



Dikes of the Keweenawan in the Huro- 

 nian show a marginal zone where the grain 

 appears to have been formed in the first 

 period of solidification, and a central belt 

 where the solidification appears to have 

 been in the third period. Throughout, the 

 augite follows the theoretical laws most 

 sharply, while porphyritically formed com- 

 ponents are less tractable. 



In illustration the speaker showed the 

 results of experiments with sulphur and 

 with candy, and found that they corre- 

 sponded to his propositions. Slides were 

 passed around, made from luster-mottled 

 melaphyrs to bring the points out. (The 

 abstract of Dr. Lane's paper is somewhat 

 obscurely worded under (a) , and it is possi- 

 ble that the undersigned has not accurately 

 expressed it. The direct application lies 

 in this: If we can measure in a diabasic 

 dike or sheet the distance of the zone of 

 even grain from the walls, then, knowing 

 as we do the fusing point of augite, we can 

 calculate the initial temperature of the dike 

 or sheet at time of intrusion.) 



In the concluding paper of the session 

 Prof. Emerson described some peculiar phe- 

 nomena that he had observed in the trap 

 sheets of the Connecticut Valley. In cer- 

 tain flows, mud in streams and chunks bad 

 become so involved that it had made the 

 trap a mass of glass and sediments. Steam, 

 supposed to have been evolved from wet 

 rocks under the lava flow by its heat, had 

 boiled up through the lava and made other 

 curious mixtures. These and the altera- 

 tions resulting were described and illus- 

 trated by thin sections. 



