GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 507 



hyphen convenient. In Tables II and IV the same analyses 

 are repeated in the fashion of presentment we propose. In 

 stating silica percentages the quantity is given to the 

 nearest unit, no fractions being used. We might have 

 written the figures for alumina in the same way without 

 exceeding the 5 per cent, limit, and would have done so 

 had the quantities been somewhat larger ; but, as they 

 range down to about 13 per cent., it has seemed better to 

 group them with the figures for ferrous oxide, magnesia 

 and lime, in statement to the nearest half-unit. The 

 remaining constituents, which generally form less than 5 

 per cent, of the whole rock, we state correctly to the first 

 decimal place. We have found that the use of the full 

 point in its normal position, as is frequently done for 

 decimals in America, and not raised by turning, as is 

 general in English practice, saves the eye from some 

 distraction, and enables more rapid apprehension of the 

 numbers. The analyses have also been rendered strictly 

 comparable by the estimation of water and carbon dioxide, 

 which are practically wholly the result of decomposition 

 and by subsequent calculation to 100. 



The composition of the Prospect rocks is such that they 

 stand on the borders separating a number of groups. They 

 are very closely related to the normal olivine-gabbro magma 

 as it appears in hypabyssal intrusions, as will be seen by 

 referring to analyses Nos. I, II, and III. All of them are 

 dolerites (diabases). The first, from Scourie, Sutherland- 

 shire, (G 3*105) consists of plagioclase giving lath-shaped 

 sections, a pale chocolate-brown augite, " titaniferous 

 magnetic iron-ore," which from the plate would appear to 

 be ilmenite, perhaps with magnetite intergrown, and 

 apatite. There is no olivine or biotite. The second is from 

 the Upper Ruhrthal, in Westphalia. The third is from a 

 massive dyke in the Berwyn Hills, North Wales, which has 



