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of unconformity in each case, can give no presumption of any 

 weight as regards affinity or age." With that I agree entirely, and 

 would point out to Mr. Holmes that he has altogether misunder- 

 stood me, for I hold that the purple-grey colour accompanies an 

 unconformity (in the district under consideration), and do not 

 assume, as he supposes, that there is an unconformity because 

 some of the uppermost rocks of the Coal Measures differ in colour 

 from the main mass below. In support of my view some evidence 

 has already been given, but as it may generally be considered 

 insufficient — as it clearly is to Mr. Holmes — I will herein adduce 

 more. If colour alone had weighed with me, as Mr. Holmes 

 supposes, I should have included in the Whitehaven Sandstone 

 series (had my paper dealt with a larger area,) such rocks as the 

 sandstone at Fort Putnam, near Blencow Station, which has exactly 

 the colour of the Whitehaven Sandstone ; but, as I have elsewhere 

 shewn, the Fort Putnam rock is down among the Yoredales, so 

 that clearly colour alone is not to be relied upon in determining 

 the relative age of rocks. Other illustrations of the inadvisability 

 of relying, in scientific investigations, upon one set of characters, 

 will readily occur to most working naturalists. And here I would 

 allude to Mr. Holmes' correlation, in the same paper, of the coal seams 

 at Bullgill and Aspatria. In tracing out the identity of the seams 

 at those two places, he apparently relies entirely upon the distance 

 which the seams are apart.; in other words, because at Bullgill there 

 are two well known seams, and at Aspatria there are also two 

 seams about the same distance apart, and the lower of which is 

 known to correspond with the lower seam at Bullgill, Mr. Holmes 

 concludes that the upper seam at both places is identical. This 

 appears to me — to borrow an illustration from Botany — like 

 employing the limited System of Linnsus for the purpose of deter- 

 mining the systematic position of a particular plant, instead of 

 having recourse to the much more reliable Natural System. As 

 Mr. Holmes must very well know, from his experience as a strati- 

 graphist, there are many other circumstances of which it is 

 necessary to take note in the correlation of coal seams besides 

 their distance apart, such for example as their sections, thicknesses, 



