RESEARCHES IN THERMAL METAMORPHISM. 199 



original structures, fossils, etc., and may closely resemble the 

 crystalline limestones produced from similar calcareous sedi- 

 ments by aqueous agency alone. In many cases, however, 

 the crystalline limestones and dolomite-rocks which owe 

 their characters to thermal metamorphism are readily dis- 

 tinguished by the occurrence in them of special accessory 

 minerals arising from non-calcareous impurities in the 

 original strata. The most significant of these special 

 minerals are various silicates rich in lime or sometimes in 

 lime and magnesia. Some of them, such as anorthite and 

 the various augites and hornblendes, are well-known 

 minerals of igneous rocks ; others, such as wollastonite, 

 zoisite, idocrase, and lime-garnets, are rare or unknown as 

 constituents of rocks formed from igneous magmas. 



In a tolerably pure limestone these accessory meta- 

 morphic minerals are formed sporadically or often in nests 

 or streaks marking the places where siliceous or argilla- 

 ceous impurities were collected in nodules or bands. Two 

 or more minerals are often associated with a definite 

 arrangement. Thus, in certain beds of the Coniston 

 Limestone metamorphosed by the Shap granite (3) occur 

 nests consisting of stellate groups of idocrase crystals sur- 

 rounded by a shell of felspar, which is again bordered by 

 clusters of pyroxene-granules passing into the matrix of 

 crystalline calcite. Attention to the mode of occurrence of 

 the accessory minerals is sometimes necessary to avoid 

 mistakes as to the orio-in of a rock. The well-known 



o 



Tiree limestone is an example. This is a rather finely 

 crystalline marble containing abundant grains of salite with 

 some felspar, sphene and other characteristic metamorphic 

 minerals ; but a closer scrutiny makes it clear that these 

 minerals have not been formed in place, but occur here as 

 detrital elements. The structure of the rock and the inter- 

 esting effects of pressure on it have been described by 

 Bonney(i4). The grains of salite, etc., may have been 

 derived from some pre-existing metamorphosed limestone. 

 Sollas and Cole (15) have suggested that they may repre- 

 sent a volcanic sand mingled with what was once a coral 

 sand. 



