136 METAMOEPHIC EOCKS 



assumed to be sedimentary. The changes in these cases are rarely 

 purely physical, though the chemical alteration may be small. 

 The ultimate composition of a rock may remain essentially the 

 same, while the method of combination of its various elements 

 has undergone extensive alteration. Quartzes and feldspars 

 may be crushed and distorted, drawn out into lens-shaped and 

 variously elongated forms, while secondary minerals like feld- 

 spars, quartz, zoisite, garnet, hornblende, epidote, and the micas 

 are abundantly generated. 



One of the commonest results of pressure effects upon igneous 

 rocks is the conversion of augite or other minerals of the pyrox- 

 ene group into hornblendes. The coarse hypersthene gabbro 

 occurring about Baltimore is found locally altered into a rock 

 consisting essentially of a schistose aggregate of hornblende and 

 plagioclase feldspars, or what, on mineralogical grounds, might 

 be classed as a diorite.^ The chemical composition in this case 

 has undergone no appreciable change; there has been simply a 

 molecular rearrangement of the particles. In such cases proof 

 of the character of the change that has taken place is usually 

 found in the fractured and otherwise distorted condition of many 

 of the constituent minerals, as well as intermediate stages of 

 alteration, whereby a residual augite crystal is found enclosed 

 in an envelope of secondary hornblende, as shown in Fig. 1, on 

 p. 36. To the secondary minerals formed in this way the tech- 

 nical name paramorpJiic is applied. To such changes as are 

 above described the name dynamic metamorphism is given. 



The protrusion of a mass of molten matter into the over- 

 lying strata may give rise to a series of changes differing from 

 the last in that they are due mainly to heat and to the chemical 

 action of accompanying vapors and solutions. Since these 

 changes are confined to limited areas along the line of the 

 contacts between the two bodies, they are defined as contact 

 metamorpMsms, 



A common form of metamorphism is manifested in the pro- 

 duction of a quartzite from siliceous sandstone. This, in its 

 simplest form, is brought about by a secondary deposit of silica 

 about the original rounded granules of sand, whereby the entire 

 mass is converted into an aggregate of quartz crystals, the out- 

 lines of which are more or less imperfect through mutual in- 

 terference in process of growth. The microscopic structure of 



^ Bull. 28, U. S. Geol. Survey, 1886, 



