230 Duncan, Wilkes and Hutchinson — On the 



About twenty rods above this dike, toward the Summit 

 station, is another dike with the same dip and strike, but 

 wholly different in appearance from the first dike. This dike 

 corresponds to Dr. Hawes' diabase dike No. 1. It has large 

 amygdaloid cavities which are filled with chlorophane (?), calcite, 

 stilbite and apophyllite. The rock is much finer grained than 

 the first dike and shows no porphyritic crystals. The micro- 

 scope shows it to consist principally of augite crystals, with a 

 small proportion of hornblende and biotite in a feldspar 

 magma. The feldspar is not well crystallized. The rock is 

 badly decomposed. 



In the dike first referred to as being similar to Dr. Hawes' 

 diorite, the microscope shows that there are present a few 

 crystals of pyroxene. This makes the two dikes at the Sum- 

 mit the reverse of each other. The writer feels more certain 

 of the identity of these two dikes with the diorite and diabase 

 of Dr. Hawes since he (the writer) made a partial chemical 

 analysis of the Summit rock which agreed almost exactly with 

 the analyses published in the papers cited above. 



I have not time to make a thorough microscopic study of the 

 above rocks, but shall be glad to furnish material to any one 

 who wishes to undertake it. I have no doubt, however, that 

 such a study would only confirm the present opinion that the 

 Summit dike is identical with the Camptonite. 



Laboratory State Geol. Survey of New Jersey. 



Art. XXXIII. — A Determination of the value of the B. A. 

 unit of Resistance in Absolute measure, by the method of 

 Lorenz ; by Dr. Louis Duncan, Gilbert Wilkes and 

 Cary T. Hutchinson. 



The work here reported upon was done at the Physical 

 Laboratory of the Johns Hopkins University during the spring 

 of 1888. 



Lord Rayleigh's modification of Lorenz's original method 

 was used : in this, as is well known, a measured part of the cur- 

 rent flowing through the inducing coils is balanced by the cur- 

 rent induced by the rotation of the disc. 



The apparatus employed is that designed by Professor Row- 

 land for his determination of the ohm undertaken for the 

 United States Government. A detailed description of it is 

 contained in his forthcoming report, so only a few words will 

 be given to it here. The induction coils, four in number, 

 were wound in square channels cut in heavy flanges, cast on the 

 exterior of a hollow brass cylinder open at both ends. The 



