128 DR. A. H. COX; REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
the magnetic disturbances be correct, will Include Coal Mea&dfes. Thus the fault 
may be an anticlinal fault, and the anticline may be the southern termination of the 
concealed coalfield. 
Although this investigation has shed some light on the structure of a concealed 
coalfield, it will be obvious that magnetic disturbances cannot be taken, without 
other geological evidence, as proof of the existence of a concealed coalfield; nor, on 
the other hand, is a coalfield necessarily accompanied by magnetic disturbance. 
It is clear, however, that the investigation of magnetic disturbances is a promising 
method of probing into the difficulties of underground structure. The elucidation of 
underground structure is not only one of the most fascinating branches of geological 
research, but it is at present one of the most pressing lines of enquiry from an 
economic as well as a scientific point of view. The necessity of such knowledge is 
brought home every day in the problems connected with the structure of the coal¬ 
fields, whether they crop out at the surface or are concealed by a cover of newer 
rocks. Our chief sources of information are based on geological mapping supple¬ 
mented by, such deep borings as have been made. The results obtained from the 
present joint magnetic and geological research show that there may be other methods 
of attacking the problem, and that an extension of these methods to other parts of 
the country is likely to yield information of considerable national importance. 
ADDENDUM. 
Peteolooy of the Igneous Kocks, 
1. The DoJerites. 
Petrologically the Nottinghamshire and Leicestershire dolerites have many features 
in common which suggest that all the intrusions belong to one and the same period. 
They are ophitic to sub-ophitic or granular olivine-dolerites carrying a purplish 
pleochroic augite and a certain amount of analcite. They show affinities, therefore, 
to the teschenites, using the term in its wider sense. If redescribed they would 
probably be compared with the crinanites or analcite-olivine-dolerites of Argyleshire* 
so far as petrological characters are concerned, though not necessarily so as 
regards age. 
The Whitwick dolerite has been described by Sir J. J. H. TeallI and by Dr. 
J. S. Flett, who remarks:—“The rock (E. 890) is a fresh and very characteristic 
ophitic olivine dolerite. The olivine is in numerous rounded grains which are passing 
into serpentine along their edges and in their centres. The augite of purplish-brown 
colour forms large ophitic plates, enclosing many lath-shaped plagioclase felspar. 
* J. S. Flett, “ The Geology of Knapdale, Jm-a Mem. Geol. Survey,’ 1911, p. 117. 
t ‘Brit. Petrog.,’ 1888, p. 211. 
