134 DR. A. H. COX; REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
3. Camjytonites Intrusive into Cambrian. 
The petrology of these rocks has been described by Sir Jethro Teall* * * § and by 
Prof. Watts.! 
The specimen tested is a medium to fine-grained rock originally of ophitic texture. 
The dominant felspars consist of cloudy albite giving small lath-shaped sections more 
or less idiomorphic. Some orthoclase is also present. The rock is much decomposed, 
and the original dark minerals, which must have been abundant, are entirely replaced 
by chlorite and calcite. A small amount of quartz is present, but is probably of 
secondary origin. Sphene is abundant in small granules scattered through the other 
minerals. It also is probably of secondary origin. Iron-ores are abundant in 
highly irregular granules, which probably consist in large part of ilmenite, and 
the susceptibility is low—21 x 10"^ The rocks were originally known as diorites, 
but Prof. Watts showed that from their characters they should be grouped with the 
lamprophyres. The example described above appears very similar to certain 
lamprophyric rocks known as cuselites. 
4. The Igneous Rocks of Charmvood Forest. 
(a) T'he (TratugyJujres have been described on various occasions by Sir Jethro 
Teall,| and by Prof. Bonney and Mr. IIill,§ so that a brief reference will sufiice. 
Two main types are recognised—an acid and a basic type. The rocks are rather 
coarse-grained comparable with granite, and are now considerably decomposed. 
They consist of idiomorphic plagioclase and augite, which has been altered to 
chlorite, epidote, and actinolite, set in an abundant granophyric ground-mass of 
quartz and orthoclase. The character and grain of the intergrowth varies consider¬ 
ably. I’he iron-ore builds rather large masses scattered irregularly through the 
ground mass. They show partial alteration to leucoxene and evidently consisted 
originally of ilmenite, not magnetite. Hence the low susceptibility of the rock as 
compared with the Mount Sorrel granite. 
The more basic type, found in the northern part of the Forest, is similar in general 
characters, differing mainly in the degree of acidity and in the relative proportions of 
the vaadous minerals and in being still more decomposed. The quartz and orthoclase 
are reduced in amount and the graphic intergrowth is finer grained. Plagioclase and 
the dark minerals are present to a greater extent but are completely altered with the 
resulting development of a lai’ge amount of chlorite and epidote. The rocks present 
certain affinities to the quartz dolerites, and apparently represent a transitional type 
* ‘British Petrography,’ 1888, p. 2-50. 
t ‘ Proc. Geol. Assoc.,’ vol. XV. (1898), p. 394, 
I ‘ British Petrography,’ 1888, p. 270. 
§ ‘ Q. J. G. S.,’ vol. XXXIV. (1878), p. 217 ; and vol. XLVH. (1891), p. 84. 
