86 the microscope. June, 



us, of the early days of this earth's history, when the temper- 

 ature near the surface was still very high. Since that time the 

 zone for marked mineral ogical changes has been continually 

 sinking until at the present day it has reached a depth practi- 

 cally unattainable. " The subterranean laboratory still exists, 

 but the way to it was virtually closed at a comparatively early 

 period in the earth's history." 



In conclusion the author considers that the progress made 

 since the microscope was pressed into the service of geology 

 augurs well for the future, and inspires the hope that we shall 

 at least learn something of the history of the earliest ages, 

 " when the earth had but lately ceased to glow, and when the 

 mystery of life began." — Jour. R. M. S. 



■ooo- 



A NEW MACHINE FOR CUTTING AND GRINDING 

 THIN SECTIONS OF ROCK AND MINERAL. 



BT GEORGE H. WILLIAMS, BALTIMORE, MD. 



A brief description of a new machine for cutting and grind- 

 ing rock sections in which electricity furnishes an economical 

 and satisfactory motive power, may prove acceptable to the 

 constantly increasing number of workers in mineralogy and 

 petrography. This machine was devised for the petrographi- 

 cal laboratory of the Johns Hopkins University over a year 

 ago, and since then it has been in more or less constant use. 



It has thoroughly established its reputation for accurate 

 and rapid work, while experience has suggested some im- 

 provement on the original model. 



The machine consists of a substantial table carrying in its 

 lower part the electric batteries and motor, while upon its up- 

 per surface is placed the apparatus for grinding and sawing. 

 The table is approximately three and one-half feet square and 

 two feet nine inches high. It is constructed, with all of its ap- 

 purtenances, by the Donaldson-Macrae Electric Company (of 

 215 No. Calvert Street, Baltimore), whose storage batteries and 

 electric motors are well known. The price of the machine 

 complete is $130, consisting of the following parts : 1. Three 

 200- Ampere hour storage batteries, 13 inches high, in portable 

 rubber cases. These batteries stand on a firmly-constructed 

 cross-piece from which they may be readily removed for re- 

 charging. 2. One one-eighth horse-power electric motor of 

 the Donaldson-Macrae pattern. This is fastened to a second 



