S. AUport — On the Midland Basaltic Rocks, 101 



Carboniferous Limestone, and was evidently a contemporaneous lava- 

 stream, partly scoriaceous, and partly of a compact Ijasaltic structure. 

 Generally tlio cavities have been lillcd with carbonate of lime or 

 other minerals, thus forming an amygdaloid ; other specimens, how- 

 ever, have empty cavities, and are as vesicular as recent lavas. A 

 specimen taken from the tunnel through the High Tor at Matlock, 

 is quite undistinguishablo from the Stafibrdshire rocks, and contains 

 a considerable proportion of Olivine. 



With respect to the precise age of these rocks, there is no satis- 

 factory positive evidence, except in Derbyshire, where they clearly 

 alternate with the Carboniferous Limestone, but there is a considerable 

 amount of negative evidence to show that all are probably contem- 

 poraneous with the Coal-measures as a whole. 



In South Stafibrdshire the " Green Kock " runs very regularly for 

 considerable distances between the same seams of coal or shale, but 

 occasionally penetrates higher beds, or, may be, has descended from 

 higher to lower. In such cases the rock is clearly intrusive, but the 

 disturbance is confined to a portion only of the Coal-measures, and 

 in some pits the singular intermixture of coal or shale with the trap, 

 at once suggests the idea that the igneous matter was intruded 

 before the consolidation of the beds. No igneous rock has yet been 

 found in the lowest beds of coal or ironstone, or in the underlying 

 Upper Silurian Rocks. In other words, there is no evidence to 

 show that it has been intruded from below in this district, nor am I 

 aware of any which proves it to have been injected subsequently to 

 the completion of the entire Carboniferous series. 



As the small Coal-fields of the district are surrounded on all sides 

 by rocks, of Permian or Triassic Age, in which no intrusive rocks 

 are to be found, there can be no reasonable doubt that the basaltic 

 rocks now described are at least Ante-permian, and that probably 

 they belong to the Carboniferous period itself. That it was a period 

 of great igneous activity is certain, from the facts so well brought 

 out by Mr. Giekie and other Scottish geologists, and as there is now 

 no doubt as to the identity in composition of all these rocks, the 

 greater part of them may well have been derived from the same 

 source, for the now detached Coal-fields are probably but the frag- 

 ments of one of far greater extent. 



It will be seen, from the description now given, that these rocks 

 correspond in every respect to those known by the various names of 

 Trap, Basalt, Anamesite, Dolerite, Melaphyr; the term basalt is, 

 however, now restricted by some to rocks of Tertiary or Eecent age ; 

 a distinction, I think, to be deprecated, as no sharp line can be 

 drawn between Tertiary and Pre-tertiary rocks. There is evidently 

 no other difierence between the Tertiary and the older basaltic rocks 

 than that produced by the subjection of the latter, during enormous 

 periods of time, to the chemical action of water holding various 

 substances in solution, and modified in many cases by heat and pres- 

 sure. In this respect, the occurrence of Olivine and its pseudo- 

 morphs is especially interesting, for, till quite recently, this mineral 

 has been regarded as characteristic of the younger basalts : we have 



VOL. VII.— NO. LXX. 11 



