198 SCIENCE PROGRESS. 



seen in the Saxon greywackes and conglomerates. Re- 

 crystallised felspars play a more or less important part in 

 many highly metamorphosed arenaceous strata, which are 

 sometimes typical gneissic rocks of somewhat coarse texture. 

 The Silurian gneisses of the New Galloway area (i) have 

 been in part grits and flags rather than shales. 



In his valuable report on the igneous rocks of Arkansas, 

 the late J. F. Williams (13) described some very curious, 

 contact-phenomena in the siliceous rocks of Magnet Cove, 

 a district which has long- been known to mineralogists as 

 the locality of several interesting mineral species and 

 varieties. The strata in question are the well-known nova- 

 culites or whetstones and the sandstones associated with 

 them, and the special minerals developed are ascribed to 

 the invasion of a group of elseolite-syenites and allied 

 igneous rocks. The most interesting; mineral is brookite, 

 which occurs in brilliant black crystals coating the faces of 

 certain large crystals of quartz. It is of peculiar habit and 

 characters, and was originally described under the name 

 arkansite. Brookite resulting from the metamorphism of 

 rutile-needles has been noticed elsewhere, but the conditions 

 of its occurrence at Magnet Cove seem to be different. 

 Williams states that the unaltered strata are practically free 

 from titaniferous material, and he supposes the titanic acid 

 which builds the brookite crystals to have been derived 

 from the syenite. Good crystals of rutile have also been 

 formed, besides rutile paramorphic after brookite. Crystals 

 and rosette-like groups of haematite, which occur sparingly, 

 are also probably titaniferous. On the question here 

 raised, of an actual transference of material from an invad- 

 ing magma to the adjacent rocks, some remarks will be made 

 later. 



Some of the most instructive features of thermal meta- 

 morphism are found in connection with calcareous strata. 

 Pure limestones and dolomite-rocks, which have been sub- 

 jected to a sufficiently elevated temperature, are always 

 found to have recrystallised without chemical change. The 

 carbonates are not decomposed by heat alone. The result- 

 ing crystalline marbles have commonly lost all trace of 



