CONTACT MBTAMOEPHISM BY INTRUSIVES. 209 



There is still a kind of contact rock in which actinolite is the chief dark 

 mineral, and in which the actinolite, though in clumps, is mainly collected 

 in bands. This corresponds to the desmosites in structure, though differing 

 from them in mineralogical composition. 



These chlorite and actinolite contact rocks may be expected to grade 

 into each other, and such a gradation is shown in one specimen, in which 

 actinolite and chlorite are ^Jresent in about equal quantity. The actinolite 

 occurs in crystals and sheaves, forming the spots, whereas the main mass of 

 the slide surrounding the spots is formed of chlorite as the metasilicate asso- 

 ciated with feldspar, quartz, and some epidote. 



It would be of great interest to determine which of the contact prod- 

 ucts, the desmosites, the spilosites, or the adinoles,. represent the greatest 

 amount of metamorphism, as shown by the relations to the intruding' mass. 

 Unfortunatel}', the records of the specimens do not enable me to determine 

 this, although for other contact zones in other regions it has already been 

 determined that the adinoles are next to the contact, while the spilosites 

 (and desmosites) are intermediate between them and the clay slates. 



COMPARISON OV THE ANALYSES OF THE NOKMAL MANSFIELD CLAY SLATES AND THE CONTACT 



PRODUCTS. 



In a series of analyses designed to illustrate the chemical changes 

 which accompany the increasing metamorphism of a rock, it is of great 

 importance that the various phases, from the unmetamorphosed to the most 

 metamorphosed form of the rock, be represented. Moreover, the order of 

 succession from the unmetamorphosed to the most metamoi'phosed form of 

 the rock should be definitely known. In the present case certain pha es 

 of the metamorphosed rocks are represented, but it has been impossible, 

 owing to poor exposures, to determine in this locality the order of succes- 

 sion. This has, however, been done so satisfactorily by Lossen and othei's, 

 and the characters of each facies in the progression have been so well 

 described, that I have no hesitation, after a microscopical study of the thin 

 sections of the specimens analyzed, in presenting the series of analyses in 

 the following table as illustrative of the changes which have taken place 

 in a clay slate, in the contact zone of dolerite, in its passage to spilosite 

 and adinole. The analyses are given in the order of approach to the 

 dolerite as determined by the character of the rocks. No. 1 is the unmeta- 

 aioN xxxvi 14 



