336 KECOKIV OF .SCIENCE FOR 1887 AND 1888. 



theintense contortion of included granitic veins and by other mechanical 

 effects recojjnized in the rocks when studied by the microscope. Messrs. 

 Fox and Toall* liuve in like manner accounted for the local develop- 

 ment of actinolite schists out of the intrusive greenstones on some of 

 the outlying islands of the Tjizard. 



But some of the most remarkable results of dynamic metamori)hisni 

 are those described by menjbers of the Scottish Geological Survey t as 

 occurring in the northwest highlands of that country. In the Archreau 

 areas of this region it is shown (i) that a great series of igneous rocks, 

 including gabbros, iicridotites, paheo-picrites, and <[uartz diorites, in 

 which i)egmatite and segregation veins had formed, have, owing to me- 

 chanical movements, developed a foliated structure and become con- 

 verted into gneisses ; (ii) that basic dik(\s injected into these rocks sub- 

 sequeut to their foliation have been by similar movements converted — 

 (1) the dolerites into diorites and hornblende schists, (2) the peridotites 

 into talcose schists, {3) the microcline mica rocks into mica schists, and 

 (4) the granites into granitoid gneisses, these gneisses being still further 

 foliated by subse«iueut movements. This change in the dike rocks is 

 in i^art molecular and in part chemical. The molecular change of aug- 

 ite into hornblende has afforded the transition of diabase into diorite. 

 Where lines of movement coincide with the margins of one of the doler- 

 ite dikes it has usually happened that portions of the outer part, it may 

 be but a few inches or feet, are converted into a hornblende schist, and 

 a further stage of change is met when a broad dike is traversed by sev- 

 eral lines of shearing, in which case lenticular or eye-shaped masses of 

 diorite are formed, around \vhich curve in wavy lines beautiful bands 

 of hornblende schist. In tlie iinal stages the eyes disappear, and the 

 whole of the original dike is converted into a zone of hornblende schist 

 "consisting mainly of hornblende and secondary feldspar, with a small 

 quantity of mica. This alteration has been attended with the formation 

 of segregations of vitreous quartz, and in the extreme cases by a com- 

 plete reconstruction of the adjacent gneisses. The latter alteration in- 

 cludes a production of secondary foliations and a more or less complete 

 reconstruction of the rock, the opalescent quartz granites having become 

 elongated and also clear and vitreous ; black mica having been devel- 

 oped out of the original hornblende, a white mica out of the original 

 feldspar, and a recrystallization of hornblende in the form of actinolite 

 needles having taken place, together with a beautiful development of 

 secondary feldspars. 



Igneous rocks intrusive in Cambrian and Silurian strata are described 

 as having likewise undergone metamorphism, in one case a porpliyritic 

 felsite being converted into a ftne-grained schist, quartzite into soft 

 sericite schist, hue-grained diorites in limesto!ies being represented by 

 green hornblende and chlorite schists. Pegmatite is described as de- 



* Qiiar. Jonr. Geol. Soc, May, 1888, XLIV, p. 309. 

 t Ibid., August, 1888, xuv, p. 378, •• " 



