6 Mr. Finlay^ on the Variatiom of [Sept. 3, 



pose the rock, passing through concentric coatings quite unaltered, and 

 forming miniature black ridges through them. 



The hornblendic portion of the dyke passes gradually into a very 

 compact a/ id highly crystalline has ilt. The basalt passes into ortho- 

 clase, which in its turn merges into a mass mainly composed of augite 

 in coarse crystals. And these variations continue till the dyke 

 terminates on the west side in a vertical wall of highly crystalline 

 basalt, the beds of the adjoining stratified rocks being undisturbed, 

 though the effects of heat are visible on east and west sides for a con- 

 siderable distance Now, though there is a great probability that the 

 hornblendic rock referred to above is a later injection into the main 

 dyke, the conditions under which it was formed must have been 

 peculiar, as the gradations from trachyte on the one side and basalt 

 on the other are perfect. The same inference cannot be made with 

 regard to the other variations in the dyke, which in many places show 

 a tendency for the particles of a single mineral to aggregate themselves 

 together over considerable areas, and thus increasing the difficulty of 

 finding any name sufficiently comprehensive and yet distinctive for a 

 dyke of this magnitude and condition. 



A few broad terms in connection with erupted rocks fit for a 

 geologist to handle in the field is a great want. Mineralogy gives no 

 assistance ; it, of all the sciences that cluster round the broad term 

 geology is the oldest, the most pretentious, and least satisfactory. 



ON THE VAEIATIONS OF LEVEL OF THE CAPE TEANSIT- 



CIEOLE.— By W. H. Finlay, B.A. 



[Eead 1884, Sept. 3.] 



The Cape transit-circle was erected in 18oo and brought into 

 regular use in 1856. From that time to the present date a continuous 

 series of observations has been maintained for determining the instru- 

 mental adjustments. 



The instrument is one of the largest of its class and is exactl}' 

 similar to the Greenwich one. The piers consist of a few large blocks 

 of very hard sandstone from a quarry near Tiger Berg, and rest on 

 the rock ; every care was taken to cement them into two solid masses. 

 It was expected with such a strong and massive instrument that the 

 changes in the adjustments would be very slight indeed, but this 

 expectation was not fulfilled. 



For the first three or four years the instrument was fairly steady, 

 but from about 1860 up to the present time there have always been 

 large and well-marked changes of position in the course of the year. 



This is more especially the case with the level error, and a constant 

 and continuous watch has consequently been kept on it. 



The telescope cannot be reversed on its bearings and is not adapted 

 for the application of an ordinary spirit level ; but the level error and 

 zero of the vertical circle (or Nadir-point reading) are determined by 

 observing the images of the wires reflected from a trough of mercury, 

 an observation admitting of extreme nicety and accuracy ; the error 

 of azimuth is determined by observations of polar stars. 



