380 A. C. LANE — STUDIES OF GRAIN OF IGNEOUS INTRUSIVES 



Explanation of Plates 



Plate 54. — Curves of Cooling — Margin Temperature Constant 



This shows the cooling of an igneous sheet, the margins of which are kept at 

 a constant temperature, taken as 0° and represented by the bottom line of the 

 diagram. The temperatures are represented by ordinates, and three different 

 scales of temperature are given, corresponding to different values of the initial 

 temperature. The main scale at the left is the same as used in the Isle Royale 

 report. To the right is the scale if, as is assumed by Queneau, Mo = .8862, and 

 also if the initial temperature of the igneous sheet (that is, its excess of temperature 

 over the surrounding rock) is taken at 1. The abscissas to the right represent tlie 

 lapse of time, the main scale at the bottom being proportional to dec. log. q and /, 

 and the scales above being proportional to mo and q, which are defined in the text. 

 Curves for twelve points from the center to the margin are drawn in full lines, 

 and in dotted lines two curves still closer to the margin, one twenty-fourth and 

 one forty-eighth of the distance to the center respectively. Also for curves 12, 4, 

 and 2 we indicate by dashes the points Mdiere the curves of the approximate solu- 

 tions which we have used in practice leave the curve which we have taken them 

 to represent, to show how small the error is which we make in using them. 



