A. HARKER THERMOMETAMORPHISM. 19 



In the metamorphism characteristic of the Lake district, the chemical changes 

 involved in the production of the new minerals are of various degrees of complexity. 

 There may be simple paramorphism, as when chalcedonic silica filling cracks in 

 the lavas is converted into crystalline quartz, still retaining in some cases its char- 

 acteristic mammillary structure. There may be mere dehydration, as perhaps in 

 the almost universal formation of brown mica from the chloritic decomposition 

 products of pyroxene, etc. To convert a substance of the nature of delessite into 

 biotite would require little more than the elimination of most of the water. Such 

 changes as these are found to be among the earliest results of the metamorphic 

 action. Again, part of the new-formed feldspar in the altered volcanics seems to 

 arise from the regeneration of original feldspar. This is well seen in the porphy- 

 ritic crystals in the lavas and in those scattered through some of the ashes, the old 

 turbid feldspar substance being replaced, partially or wholly, by new pellucid 

 material ; but the pseudomorphs no longer consist of single individuals, and one 

 cannot positively assert that they are chemically identical with the original feld- 

 spar. The other new-formed minerals for the most part indicate atomic rearrange- 

 ments of a more complex order. 



The minerals generated in the metamorphism of the volcanic rocks are numerous, 

 at least in the basic and intermediate groups. Most important in the list are quartz ; 

 various feldspars ; biotite and allie dmicas ; green hornblende, actinolite and tremo- 

 lite ; a lime-augite ; sphene, rutile and ihnenite ; magnetite, pyrite and pyrrhotite. 



In all the volcanic rocks in their most highly metamorphosed state a large pro- 

 portion of the bulk is found to consist of feldspars, among which are recognized 

 orthoclase, albite, anorthite, and some of the intermediate varieties. With this 

 constant abundance of new-formed feldspars we may correlate the absence or rarity 

 of certain aluminous silicates, such as garnet, andalusite, staurolite, etc, known as 

 common metamorphic minerals in many sedimentary rocks. Cyanite and andalusite 

 occur only occasionally in some of our metamorphosed ashes, and the garnets are 

 entirely wanting. Such minerals will naturally arise in the metamorphism of rocks 

 impoverished in alkalies by the ordinary processes of chemical degradation ; and, 

 in contradistinction to these, the abundant formation of feldspars may be expected 

 to characterize the alteration of rocks of direct igneous origin. Feldspars, however, 

 are certainly formed in many metamorphosed sedimentaries, either in addition to 

 andalusite, etc, or to the exclusion of such minerals whenever the original material 

 contained sufficient alkalies. The metamorphism of certain flags near the Shap 

 granite has given rise to abundant feldspars, while garnet and chiastolite are absent 

 and andalusite is certainly n< >t characteristic. So far as our data go, this seems to be 

 a more common thing in the older than in the newer sediments. Broadly speaking, 

 we may expect the newer detrital rocks, in so far as they are derived from older 

 sedimentaries, to become increasingly poor in alkalies.* The apparent reluctance 

 of some geologists to admit feldspar as a highly characteristic product of extreme 

 thermometamorphism may be due to the fact thai the minuteness of the grains in 

 most cases, the rarity of twinning, and the singular clearness of the mineral make 

 it often easily mistaken for quartz. It is instructive to compare the ultimate 

 destruction of the feldspars in extreme dynaniometamorphism. 



♦ Taking at random the analyses of " Thonsehiefer " given by Both, rejecting only those cases in 

 which the stratigraphy is known to beat fault, I find that twenty-one examples grouped under 

 «' Silur " give average percentages 3.864 of potash and 1.226 of soda; twentj seven under " Devon " 

 mid " Culm" give 2.701 of potash and 0.973 of soda. 



